WIP: Search API

DO NOT MERGE:

Leaving this PR open in case I don't come in, this is where I stopped... you should be able to pick up work from this PR.

Current status:
* doing TDD in `SimpleSearchApiTest`
* top-level term filters are working
* top-level facets are working
* nested filters aren't working - may be an ES/Elastica index config problem
* nested facets aren't working - probably config problem
* needs docs in the controller, and search models

Try deploying to staging - if it works, provision a new machine specifically for search, and configure/deploy accordingly.  The search machine should only need ES, nothing app specific.  The search_index rabbit consumers should run on the task machine.