First step of converting db use to go through the PeeWee ORM library.

This is the first incremental step of converting SickBeard to use an ORM for it's database.  When finished it would allow mysql or postgres (in addition to sqlite) to be used as the DB.

This pull request adds the base models, tests and converts tv.py to use the new models.

This is my first time using github so let me know if I'm doing anything wrong.