Add support for stats

The statistics from Couchbase can now retrieved via

    # default stats
    stats = cb.stats()
    # tap specific stats
    stats = cb.stats('tap')

Change-Id: Icc2b7cd71a6c4c69267d593fc1dea4f500cb1a58
Reviewed-on: http://review.couchbase.org/25876
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Mordechai Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>