Task history, implementation of https://github.com/spotify/luigi/issues/51

All the tables are added as per the RFC, though currently
only Task, TaskEvent, and TaskParameters is being used right now.

Added a simple HTML front-end using Flask. I'd be open to using tornado
to be consistent with server.py, I just used flask because I've used
it before.
Removed test tasks
Guards imports of sqlalchemy, flask, update travis