Added some api endpoints to user model.
Added destroy statuses method to status model. Added me method to api.
Added direct message endpoints.
Implemented friendship endpoints.
Added friends/followers get ids endpoints.
Added account endpoints. Still need image update support.
Added favorite methods. create/destroy favorites not working yet.
Added simple example.
Minor update.
Added method to set credentials.
Added support for callbacks.
Fixed error in api.py
Revert "Added support for callbacks."

This reverts commit 58ee4790515e99f1486f63ff30dcbf6fe0ce11c2.
Exposed TweepError
Added notifications enable/disable methods.
Added create, destroy, exists blocks methods.
Implemented blocks() and blocks_ids() methods.
Implemented saved searches endpoints.
Implemented help/test endpoint. Condensed parsers that had duplicate code.
Added global, unauthenticated instance of API class into package init module.