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