Improved health checking for RiakNode.

We now can execute operations during the healthcheck. Two are provided
currently: ping (default) and Store.

The exsiting RiakNodeFixtureTest exercises the behavior.

I also bumped Netty to the latest final in the pom.

Resolves #341
Added constant for heckcheck codec.
Merge pull request #398 from basho/gh341-improve-health-check

Improved health check in RIakNode