Pushing docs towards Python 3

[rafaelsierra]

It is not clear from the docs that there is a difference in the returned value between Python 2 and Python 3 (#177 ), so I made a few changes that would make it clear that on Python 3 return value is actually composed of bytes instead of a string.

I did not bother with specifying that Python 2 returns a string mainly because it is being retired by the end of this year. Feel free to push changes that would make clear the distinction between python 2 and 3 return values.