Extendable Criteria object for building DSLs to find particular entities

[shabbyrobe]

Splitting discussion from #14 into separate discussions. Paging @dig412.

I've got something similar that I've been working on for this as well, but it's not ready for prime time. I'll clean it up a bit and post it as a gist some time in the next week or two. It might be interesting to see if we can combine the ideas we've had separately.