Add `quiet` parameter to Connection

When `quiet` is set to `False` (default), accessing
a non-existent key would raise an exception. If set
to `True` it will return `None` instead.

Change-Id: I5894e3ddbf01ec6047bfd174d13ce807f82e0f19
Reviewed-on: http://review.couchbase.org/25763
Reviewed-by: Mordechai Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>