Rename Connection to Bucket

Note that there are still likely lots of doc references pointing to the
older name. This commit also includes a dummy subclass of Bucket called
Connection so that existing code will not break.

Change-Id: I22dc8564d9d009567a7e940c4e981a82372f6c6c
Reviewed-on: http://review.couchbase.org/41307
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>