Remove 'TxCouchbase' class

This *BREAKS* older code (though not any release code) which uses the
'from txcouchbase import TxCouchbase'. This code will now need to be
changed to use 'from txcouchbase.connection import Connection as
TxCouchbase'

Change-Id: I30b6524342e364e237d30946e4f30bf51119518c
Reviewed-on: http://review.couchbase.org/30875
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>