Fixes #193, fixed when Warning is raised, fixed error raising, version bump

- Added `get_retweeters_ids` method
- Fixed `TwythonDeprecationWarning` on camelCase functions if the
camelCase was the same as the PEP8 function (i.e. ``Twython.retweet``
did not change)
- Fixed error message bubbling when error message returned from Twitter
was not an array (i.e. if you try to retweet something twice, the error
is not found at index 0)