fix python3 compatibility

[b-ryan]

closes #66, #118, #137, #140, #192, #206

^ This seems to be the most pressing issue to resolve. I'm hoping to fix python3 compatibility once and for all in this branch. Looking for feedback from anyone who knows more about this stuff.

Notes on the changes in here:

• os.getenv and related functions return strings, not unicode (in python 2). This will cause an issue in the draw function if you do not decode the string into utf-8 and there are non-ascii characters in the string, because concatenating a unicode object and a string causes Python to try to implicitly convert the ASCII to unicode, but can't convert the non-ascii character. Here is some that demos this:

>>> x = "powerline-shell-\xe2\x9c\x8e" # bytes to make the pencil utf-8 character
>>> x + u""
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 16: ordinal not in range(128)

• In Python 3, everything is unicode. There is also no str.decode function.
• However, in Python 3, the subprocess functions return bytes instead of unicode. Thus we must always decode the bytes into unicode. This works just fine though since in Python 2, subprocess returns strings which must also be decoded into unicode.

Tests:

• cd into directory with non-ascii character in its name (py2)
• cd into directory with non-ascii character in its name (py3)
• git branch has unicode char (py2)
• git branch has unicode char (py3)
• hg basic test (py2)
• hg basic test (py3)
• basic jobs test
• basic ruby/node version tests

[b-ryan]

Did some pretty extensive testing. Only thing I left out was testing the SVN changes. That I'm just hoping I'm not missing something.

[b-ryan]

Just gave SVN a quick run and found no issues. I think this change is ready to go.

[LeonardoGentile]

Hello, this seems not to work for me with pyenv.
pyenv global 3.4.3 shows no prompt at all, all the segments disappear but no error is showed.
If I pyenv global 2.7.6 then things go back to normal