[NO CLUE WHAT I'M DOING] Add an Elasticsearch engine

Summary:
I have no idea what I'm doing, but here's part of an elasticsearch engine. These things work:

• Indexing stuff (??)
• Searching for text/type?
• Reconstructing things??

All the complicated stuff doesn't work. I'm having a hard time figuring out the best way to model things because elasticsearch's documentation is not exactly the most complete or illuminating.

@amckinley, does this look sane-ish so far? Particularly, the /phabricator/<type>/<phid>/ URI scheme and how I've set up the relationships and fields in the documents?

How should I model the relationship and field queries? I want, like, an "equal" query but it seems like I've got "text" or "term" to work with and neither are exact match? And "term" doesn't consider PHIDs to be terms since they have hyphens in them?

I'll keep kind of slogging my way forward here but if you have valuable wisdom to share it would probably get me to a better end state much faster. The whole query construction phase is pretty much black magic to me.

Test Plan: nyancat

Reviewers: amckinley, vrana

Reviewed By: vrana

CC: jungejason, tuomaspelkonen, aran, 20after4, vrana

Differential Revision: https://secure.phabricator.com/D790