Rename the default / master / test development SQL file

Connected to issue #24, I think we should rename the sql file that will sit in the `db/` folder to `wordpress.master.sql`

The naming convention going forward will be;

`{database_name}.{timestamp}.sql` for the backed up versions of existing databases

`{database_name}.master.sql` for the latest **master** copy which will be updated and synced to the remote version