Expose 'lcb_t' pointer attribute

This is useful for FFI integration

Change-Id: I615a4255867203e1a6ad28a672b56af45d37b111
Reviewed-on: http://review.couchbase.org/29725
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>