#29 Validate DB schema in `check_database`

`table_exists` will only check if there's a table with that name, and returns true even if columns are wrong. This is okay for now since I don't see how a table without the right columns could happen except if the user fucked with the DB manually, in which case he should know enough to fix it himself. This will be a real problem if you ever change the schema in later versions. When that happens, use something like `peewee-validates` to validate the schema.