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]

setup.py

@@ -25,7 +25,7 @@ setup(
     include_package_data=True,
 
     # Package dependencies.
-    install_requires=['requests==1.2.0', 'requests_oauthlib==0.3.1'],
+    install_requires=['requests==1.2.1', 'requests_oauthlib==0.3.1'],
 
     # Metadata for PyPI.
     author='Ryan McGrath',