Continuous integration with Travis and Docker, fixes kivy#625

This is a first step to python-for-android continuous integration.
It currently only verifies that p4a creates an APK by cross compiling
a limited set of recipes.

How it works; On git push Travis "executes" `.travis.yml` instructions:

1. the `before_install` section runs `docker build` to prepare the environment
2. the `script` section is the actual test, building the APK with `docker run`
3. based the exit status Travis build will be green or red

For example editing `pythonforandroid/recipes/hostpython2/__init__.py` and
introducing an error e.g. replace `-j5` with `--wrong-flag` would make it fail.

Things to improve:

 - improve `.travis.yml` readability
 - test more recipes
 - bring Python3 and Crystax support
 - speed up build/run by caching downloads
 - and much more

.travis.yml

@@ -0,0 +1,14 @@
+sudo: required
+
+language: generic
+
+services:
+  - docker
+
+before_install:
+  - docker build --tag=p4a .
+
+script:
+  - docker run p4a /bin/sh -c 'uname -a'
+  - docker run p4a /bin/sh -c '. venv/bin/activate && p4a apk --help'
+  - docker run p4a /bin/sh -c '. venv/bin/activate && p4a apk --private testapps/testapp/ --package=com.github.kivy --name "Testapp" --version 0.1 --sdk_dir /opt/android/android-sdk/ --android_api 19 --ndk_dir /opt/android/android-ndk/ --ndk-version 16b --bootstrap=sdl2 --requirements=python2,kivy'

Dockerfile

@@ -0,0 +1,67 @@
+# Dockerfile with:
+# 	- Android build environment
+# 	- python-for-android dependencies
+# Build with:
+# docker build --tag=p4a .
+# Run with:
+# docker run p4a /bin/sh -c '. venv/bin/activate && p4a apk --help'
+# Or for interactive shell:
+# docker run -it --rm p4a
+#
+# TODO:
+#	- delete archives to keep small the container small
+#	- setup caching (for apt, pip, ndk, sdk and p4a recipes downloads)
+FROM ubuntu:16.04
+
+
+# TODO
+# wget --quiet --output-document=sdk-tools.zip "https://dl.google.com/android/repository/sdk-tools-linux-${ANDROID_SDK_TOOLS_VERSION}.zip"
+
+# get the latest version from https://developer.android.com/ndk/downloads/index.html
+ENV ANDROID_NDK_VERSION="16b"
+# get the latest version from https://developer.android.com/studio/index.html
+ENV ANDROID_SDK_TOOLS_VERSION="3859397"
+
+ENV ANDROID_HOME="/opt/android"
+ENV ANDROID_NDK_HOME="${ANDROID_HOME}/android-ndk" \
+	ANDROID_SDK_HOME="${ANDROID_HOME}/android-sdk"
+ENV ANDROID_NDK_HOME_V="${ANDROID_NDK_HOME}-r${ANDROID_NDK_VERSION}"
+ENV ANDROID_NDK_ARCHIVE="android-ndk-r${ANDROID_NDK_VERSION}-linux-x86_64.zip" \
+	ANDROID_SDK_TOOLS_ARCHIVE="sdk-tools-linux-${ANDROID_SDK_TOOLS_VERSION}.zip"
+ENV ANDROID_NDK_DL_URL="https://dl.google.com/android/repository/${ANDROID_NDK_ARCHIVE}" \
+	ANDROID_SDK_TOOLS_DL_URL="https://dl.google.com/android/repository/${ANDROID_SDK_TOOLS_ARCHIVE}"
+
+# install system dependencies
+RUN apt update && apt install --yes --no-install-recommends \
+	python virtualenv python-pip wget curl lbzip2 patch
+
+# build dependencies
+# https://buildozer.readthedocs.io/en/latest/installation.html#android-on-ubuntu-16-04-64bit
+RUN dpkg --add-architecture i386 &&  apt-get update && apt install --yes --no-install-recommends \
+	build-essential ccache git libncurses5:i386 libstdc++6:i386 libgtk2.0-0:i386 \
+	libpangox-1.0-0:i386 libpangoxft-1.0-0:i386 libidn11:i386 python2.7 python2.7-dev \
+	openjdk-8-jdk unzip zlib1g-dev zlib1g:i386
+RUN	pip install --upgrade cython==0.21
+
+# download and install Android NDK
+RUN curl --progress-bar "${ANDROID_NDK_DL_URL}" --output "${ANDROID_NDK_ARCHIVE}" && \
+    mkdir --parents "${ANDROID_NDK_HOME_V}" && \
+    unzip -q "${ANDROID_NDK_ARCHIVE}" -d "${ANDROID_HOME}" && \
+	ln -sfn "${ANDROID_NDK_HOME_V}" "${ANDROID_NDK_HOME}"
+
+# download and install Android SDK
+RUN curl --progress-bar "${ANDROID_SDK_TOOLS_DL_URL}" --output "${ANDROID_SDK_TOOLS_ARCHIVE}" && \
+    mkdir --parents "${ANDROID_SDK_HOME}" && \
+    unzip -q "${ANDROID_SDK_TOOLS_ARCHIVE}" -d "${ANDROID_SDK_HOME}"
+
+# update Android SDK, install Android API, Build Tools...
+RUN mkdir --parents "${ANDROID_SDK_HOME}/.android/" && \
+	echo '### User Sources for Android SDK Manager' > "${ANDROID_SDK_HOME}/.android/repositories.cfg"
+RUN yes | "${ANDROID_SDK_HOME}/tools/bin/sdkmanager" --licenses
+RUN "${ANDROID_SDK_HOME}/tools/bin/sdkmanager" "platforms;android-19"
+RUN "${ANDROID_SDK_HOME}/tools/bin/sdkmanager" "build-tools;26.0.2"
+
+# install python-for-android from current branch
+WORKDIR /app
+COPY . /app
+RUN virtualenv --python=python venv && . venv/bin/activate && pip install -e .