use snake case for table names in the database

[kkasra12]

as we know if the tablename is not mentioned in the Meta class, the default table name will be class.__name__.lower()+'s'

but if the class name is in CamelCase, it will create a total mess in the database. apparently, there is no way to automize this process (converting CamelCase into snake_case) in the Meta class, and table names should be hard coded for each class.