Riak DT WIP

Work in progress on #166

* Implement types to support maps
* Implement fetch/update for counters and sets
* Implement fetch for sets
* Move `MapFieldName` to `RiakDtMapField`

* Counter implementation - it may be possible to migrate Riak 1.4 counter code to use the new Riak DT counter code.
* Investigate whether deserializeObject really needs a `contentType`
* Implement update for map values