Update GetMentions to API 1.1

https://dev.twitter.com/docs/api/1.1/get/statuses/mentions_timeline
Fix twitter.List AsJsonString()

AsJsonString() for a List didn't work because the 'user' entry was a
twitter.User object instead of a dict.
Update CreateList to API 1.1

https://dev.twitter.com/docs/api/1.1/post/lists/create
Update DestroyList to API 1.1

https://dev.twitter.com/docs/api/1.1/post/lists/destroy
Update GetLists to API 1.1

https://dev.twitter.com/docs/api/1.1/get/lists/ownerships