Move CouchbaseBucket and MemcachedBucket to Couchbase namespace

Change-Id: I10843acdaeece5db304a12678b303d315d0f14b4
Reviewed-on: http://review.couchbase.org/42130
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>
Reviewed-by: Simon Baslé <simon@couchbase.com>