Adding an option to disable landlord tenant controller helpers and assets resolver 

I added a function called 'is_spree_domain?' to the tenant helper. 

This function can be overritten by the user when she doesn't want to activate the spree tenant validation methods on a specific domain (or any condition, really).

Also, I added a rescue that activates runs 'super' on the assets resolver, that way if a tenant is not found, the application continues as normal.

All tests pass, but I didn't see a straightforward way of testing this specific changes looking at the existing test base.