Not ready for merge: Elasticsearch backend

Hello,

I'm filing this dummy pull request so that my branch adding elasticsearch support will be visible from the main repo.  There are four outstanding tasks before its ready for merge:

1.) mapping returned values to native types
2.) case-insensitive searching
3.) escaping metacharacters in contains regex
4.) abstract backref support

I plan on working on these this week and will hopefully have a final PR soon.

Cheers,
Fitz