Add PATCH-based update

This simplifies the way a document can be updated. It changes the syntax
to, for example:

    curl -XPATCH ":9200/foo/user/1" -d '{"first_name":"Stephen"}'

Instead of the more verbosely-nested:

    curl -XPOST ":9200/foo/user/1/_update" -d '{"doc":{"first_name":"Stephen"}}'

This provides a more RESTful interface and better analog to the PUT
method.

Signed-off-by: Stephen Celis <stephen@stephencelis.com>