Support mysql database, not just postgresql

TransitScreen's database schema is pretty simple and doesn't require postgresql.

Someone should create a version of the schema for mysql.

Eric notes in the README that

-- application/config/database.php
Here is where you set up your database connection with the appropriate
login credentials and database name. Please note that you will have
to set the correct value for $db['default']['dbdriver'], which is set
by default to 'postgre'. If you intend to use MySQL, you will need
to change this value.