Add start_at and end_at to Release
Changes to test_helper for testing gems

- Include FactoryGirl helper methods in tests
- Make sure we start the DatabaseCleaner transaction
Add a first pass at a ReleaseSlot class

The ReleaseSlot model is not backed by a database and will return 
slots in 30-minute increments for a given day. Each slot may be
associated with a Release that is taking place during the slot.
Set up chosen-jquery and associated layout code