Updating tickets (#265, #87, #181, #207, #208, #300, #387, #399, #429, #431, #438, #439, #72, #182, #193, #229, #255, #273, #311, #158, #299, #326, #380, #400)

I see that this is stubbed out in Storage.scala, so it just needs a backend API. I can work on it, if thats ok.

Comment in code says pop removes the tail of the vector. Since the add and + operations prepend elements to the vector, shouldnt the vector.pop remove the head?

Otherwise this would pretty much be a queue? Add to head, remove from tail?

Supported by Voldemort,Riak,Cassandra currently.

Other backends will throw UnsuportedOperationException on commit, and should be modified to support pop