Towards a CLI for YARN's REST API

This PR makes progress towards that CLI, but isn't quite there yet.

The reason for filing it now is that the prior approach of using a Java WS provider for the REST calls has hit a wall: JAX-WS wouldn't allow me to override the SSL checks to deal with self-signed certificates. And Apache CXF was very sensitive to classpath reorderings. 

Hence, I switched the code to us Apache HTTPClient instead. This also trims our dependency tree quite a bit. This PR contains that new code as well as some spillage from the CLI work.