T53 username update

Changes include:
- Added two attributes to TwitterUser :   uid (default 0) and former_names (default = '{}')
- Modified user_timeline to:
    1. pull timelines based on uid, not on stored screen_name
    2. check if stored screen_name no longer matches the name associated with the twitter uid.  If so, update the stored screen_name.
- Created a management command which must be run on existing database to populate TwitterUser uids based on stored screen_name