Add ping action

The official name of the `HEAD /` action is `ping` according to the [rest api spec](https://github.com/elasticsearch/elasticsearch/blob/master/rest-api-spec/api/ping.json). We currently call it `available?`.

This changes the official method name to `ping` and its notification action to `cluster.ping`. It's aliased as `available?` so existing code still works.

In the future, we may want to pull this file in during tests and use it to check api completeness. Maybe add a rake task?

We may also want to check that our notification action names are the same as the spec action names (See https://github.com/elasticsearch/elasticsearch/tree/master/rest-api-spec/api).

/cc @TwP