PYCBC-118: Provide informative __repr__ for Connection

Change-Id: Ie2b3b6be8534e18222698c0c3e108882ee20e0e8
Reviewed-on: http://review.couchbase.org/26400
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>