Provide jetty9 proxy artifact

[kolloch]

All Marathon instances currently proxy to the current leader. Marathon uses its own proxy implementation with various issues.

Jetty 9 includes support for proxying. Thus it would probably make sense to upgrade to Jetty 9 to benefit from this functionality.

Since this is functionality which would be useful to frameworks other than Marathon, I would include the proxy artifact in Chaos, potentially even some utility support to provide proxying to another host dynamically.