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]

twython/compat.py

@@ -20,10 +20,12 @@ if is_py2:
     except ImportError:
         from cgi import parse_qsl
 
+    str = unicode
     basestring = basestring
 
 
 elif is_py3:
     from urllib.parse import urlencode, quote_plus, parse_qsl
 
+    str = str
     basestring = (str, bytes)