Demodulize model class name before transforming it to table name

I found myself with a model defined inside a module and my migrations wouldn't work anymore.

That's because `name.tableize` will return `"module/class_name"` and thus generate an invalid table name for the schema.

This fix simply `demodulize` the class name before "tableizing" it, so that we don't end up with slashes in the table name.

It works for me and my tables, however I'm thinking: maybe it would be better not to demodulize and replace slashes with underscores, so that the module names become a prefix for the table name?  
I'm open to discussion ^^

As for testing this, well... I'm not sure it is satisfying enough that tests still passes after this change? I'd be glad to add specific tests but I'll happily take pointers on how/where to test it!