Add Delete cascades and unique constraints

Added unique constraints to avoid having duplicate users, beers, beer
styles, breweries and ratings. Also added deletes on cascades to remove
child entries when parent entries are removed to keep data integrity.
For part 2 of exercise do few query insertions.

Did a few insertions, an update and delete to realize the futility of
all this. Off to go write a script that will do this for me.