Storage options

Adding 2 small changes to store_options:
 * Setup `storage_options` class variable when the class is created.
 * Validate the class when calling `store_in` to make sure people are not extending a Model and changing the `store_in`.

review @durran 

If you like the idea, I am wondering the keep cleaning this up, to avoid having instance vars and hashes and multiple methods to get those storage_options.