Task history, from 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.

Some screenshots of the pages:

* Recent runs: http://cl.ly/image/1h01133g1Q07
* Task details: http://cl.ly/image/1P1i3T0C1y42
* All "A" runs: http://cl.ly/image/2X3h0X3k3P2h