Fix to address issues with handling pathlib-based paths - Closes #49

[jcohen02]

In more recent versions of Django, the django-admin startproject script creates a settings file where BASE_DIR is specified using pathlib.Path. In older versions it was set using os.path.

django-drf-filepond has some code that attempts to use string functions on BASE_DIR assuming that it is a string. However, if the startproject command was used and the default BASE_DIR setting is left in place, this will cause issues in certain circumstances.

This fix addresses these problems while also retaining support for Python 2.7 (despite being well beyond EOL, aiming to keep support for legacy apps using django-drf-filepond for now), which doesn't have the pathlib module. The fix also retainins support for Python 3.5 which does have pathlib, but where the os.path.join function, which is also used with BASE_DIR in this library, can't accept pathlib.Path obejcts as parameters.