Replace requests_timeout with general timeout parameter

This closes #112
PostMedia() now supports file-like objects

PostMedia() now accepts file-like objects with an read() method in
addition to passing the media as a filename or HTTP URL. Closes #125

Also we now close the FP after read()ing it when using a filename as
parameter.
Update to python 2.6, remove simplejson dependency

This updates the requirements to python 2.6, removing the simplejson
dependency as simplejson is included from python 2.6 onwards. Also
various things were cleaned up/corrected:

* Removed included simplejson

* Google APP Engine has also moved to python 2.7 so the import logic
  for simplejson in __init__.py was removed, as well as the unused
  urlparse imports.
  https://developers.google.com/appengine/docs/python/python25/diff27

* Cleaned up unused libraries/dependencies in documentation

This closes #123
Rewritten get_access_token.py to use requests-oauth

get_access_token.py now uses the requests oauth lib and no longer
depends on oauth2. This closes #151