Add method to get libcouchbase version

Change-Id: Ie81a7f55433a36f28de7075de976d9e6255357f4
Reviewed-on: http://review.couchbase.org/26345
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>