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)

setup.py

@@ -4,7 +4,7 @@ import sys
 from setuptools import setup
 
 __author__ = 'Ryan McGrath <ryan@venodesigns.net>'
-__version__ = '2.9.1'
+__version__ = '2.10.0'
 
 packages = [
     'twython',