Set up Trigger Model and User Trigger Model

A few notes:
1. I decided to use a has many through relationship as I believe that it's highly possible that more than one user will want alerts on the same keyword. If you are not happy with this, I can redo it.

2. I removed --warning from .rspec following this advice: https://github.com/rspec/rspec-rails/issues/1103 as I had a circular require warning from the sass gem.