txcouchbase: Don't import on __init__.py

Change-Id: Ia83dacbee3d9d0f63107d4f7cc8ab224c1185531
Reviewed-on: http://review.couchbase.org/29863
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>