Port to Jetty 9.

This is still very much a work-in-progress. Due to changes in the Jetty API from 7 to 9 not all of the features in ring-jetty-handler have been ported over yet. Please see [the tests](https://github.com/tvaughan/ring-jetty9-adapter/commit/4331baf71c03b20dc19bc4df3e62e96a18e3378d) for what's been left out.