Update requests dependency, add str py2/3 compat, __repr__ definition, removed unicode2utf8 & encode static methods, update HISTORY

@ryanmcgrath Let me know if you're okay with the removal of
Twython.unicode2utf8 and Twython.encode. I moved Twython.encode to
_encode in helpers.py (only place being used is
Twython.construct_api_url) If it's python 2 and unicode then we encode
it, otherwise return the original value

[ci skip]