Integrate merchant gem, converting a view to erb

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).