PYCBC-247: Add 'value' argument to lcb_cntl

This adds a new `value_type` parameter which is more flexible than the
previous _cntl() function which did implicit conversion based on the
Python type passed (and the cntl op being used).

While I'd really like to remove the older code there, and even though it
isn't public API per se, i'd like to keep things as compatible as
possible.

Change-Id: I09cb6d7f9d61eae777c88e9232ff84b67539a6ca
Reviewed-on: http://review.couchbase.org/38220
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>