PYCBC-193: Handle large query URIs

This converts them into POST requests if they exceed a certain length.

Change-Id: I8265427863ec479a58dd77a82fe9a89da5101b44
Reviewed-on: http://review.couchbase.org/29276
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>