Remove hardcoding of mysql adapter

Napa is somewhat mysql-centric, but it doesn't have to be. If you let the database.yml file specify `adapter: mysql`, then there is no reason to encode that into the db.rake. That will allow it to be used easily with other databases and leave the database.yml configuration to the user.
Merge pull request #49 from skwp/patch-1

Remove hardcoding of mysql adapter