:construction: Integrate merchant gem

Mustache is just too painful because you can't see half of the errors.

The almost-empty view in app/views is needed for rspec to run at all (makes me a bit sad). With some logic in the view finding code we should be able to make sure that it never gets displayed to visitors.

This brings up the question: do we test client-specific views, and how do we do it in a maintenable question? Definitely something to keep in mind as we proceed.

I'm keeping Product inside the catalog for now until I add more classes and I figure out how I want to lay them out.

I am a bit worried about the amount of setup code for the catalog, though. Any ideas in that area would be greatly appreciated (how to rearrange the whole object graph).
Just FYI - Doubles should represent what they are a double of. So `double :product` becomes `double("Product")` + no need to create symbols when they are not needed (Memory Management) :)