Implemented Store, including automatic marshalling of RiakValue to HttpEntity

The ConflictResolver is also needed for Stores (when returnbody == true) so it got moved to the Bucket level. It is also no longer implicit since it is better to make the choice of which resolver to use more explicit

RiakValue now stores a byte array, matching the binary BLOB that Riak sees it as, and has a handy asString method that converts the value to a String