Introduce search handlers to perform product search

I found Haystack's interface via forms not sufficient, and did not want
to fit into their SearchView as it's slightly different from regular
Django class-based views. Furthermore, I'd like to keep the parts of the
codebase that rely on Haystack pretty low, so a search handler that
allows non-invasive searching from a view has been built.
Add fallback search handler

The SimpleSearchHandler imitates the behaviour of ProductCategoryView
before we added the faceting support. Tests pass!