Create tests for multiple live token cases

- test to ensure that token is valid even after user requests multiple other tokens
- (pending) test to ensure that other tokens are invalidated
Use MySQL/AR for email tokens

We need to be able to query for tokens based on user and based on hashed token. Because the cache solution was getting complicated, put them in the database (for now).
remove some unused tables/columns
Expire old tokens

Setting default scope to only include tokens created in the last 2 hours.

Also added a scope to query for expired tokens.
Merge pull request #230 from 18F/feature/mysql_auth_tokens

Feature/mysql auth tokens