Add ping support

Closes #510

Few things to note here:

* Ping just executes a HEAD request to "/" and doesn't contain a response body.  We do have a RawDispatch for Ping, and so I followed the normal request/response path and implemented an `IPingResponse` which is essentially empty.  Should we break from the normal path and just return true/false instead?

* Because Ping has no body and we're following the normal RawDispatch approach, I had to add additional logic in `ElasticClient.ResultsSelector`, adding a `CreateBodilessInstance` method.  Let me know your thoughts here @Mpdreamz.