Schema fixes

Removed the on_optimize functions from the models.
We really should not be dynamically adding optimizations using reflection from `rake:run`, as these optimizations are also (intended) done in the schema, (which is what is loaded with `rake db:reset`), and are designed to be created with ActiveRecord Migrations.

Also, dropped reviewers field from the Commit model.
Fixes #76 
Requesting @andymeneely take a look at this.