Rename internal pycbc_Connection to pycbc_Bucket

Change-Id: Ib58214cc68f52d8eeed26127306cd2aa5148da4b
Reviewed-on: http://review.couchbase.org/41305
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Dave Rigby <daver@couchbase.com>