Release multiple versions to support the whims of ES features

[mboynes]

Elasticsearch has a fairly rapid release cycle, and it has no dedication to backwards compatibility. Furthermore, there's no great way to see what version of ES we're querying against (without making an additional HTTP request, and that's assuming the server supports that endpoint).

As such, we're going to split ES_WP_Query and support multiple versions at once.

See #35, #36.

[gibrown]

@mboynes if you have a WP.com VIP that you want to try out with ES 2.x we now have a 2.3 cluster deployed that as a few production indices on it. Still very much in the testing phases, but we have some internal indices and en.support.wordpress.com running on it now.