Flesh out models

[cheshire137]

Barebones models were added in #5. We should add reasonable validations and relations in a follow-up branch. Some fields are marked as NOT NULL in the database, so we should have presence validations on them. There should be length validations on string fields. Relations such as belongs_to :user on the Player model should be added to connect models together based on foreign keys.

Different models could be tackled in separate branches to keep pull requests easily reviewable.