Add support for CRDT operations

Adds `DtFetch` and `DtUpdate` to support CRDT operations.

Riak message codes are uint8 (1-255) so updated the message method to reflect this.

To run tests, install Riak 2.0 and configure appropriately:
1. Set the backend to [leveldb](http://docs.basho.com/riak/2.0.0/ops/advanced/backends/leveldb/) in `riak.conf`:
    `storage_backend = leveldb`
2. Create the `riago_dt_test` data type:
    `riak-admin bucket-type create riago_dt_test '{"props":{"datatype":"counter"}}'`
3. Activate the `riago_dt_test` data type:
    `riak-admin bucket-type activate riago_dt_test`

Let me know how this looks.