: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).
It's a double of `CnC::Merchant::Product`, actually. But I strongly feel that it's important for this test to be oblivious of that. All this test cares about is the interface.

You could make an argument that the stubs in the individual test examples should be moved up to be all in one place (so that the interface is better specified). Couldn't decide about this—the way it is now, the requirements are specified per-example). WDYT?