Spark Job Server Preview

This is a preview of our Spark Job Server which we would like to contribute back to Spark as a new submodule.
In summary, the job server allows an organization to offer Spark as a REST API Service.   No more bastion boxes, app deploys, and having to repeat common infrastructure things like serializing job results, tracking progress, setting up contexts.   The job server also makes it very easy to have multiple jobs share RDDs and a single SparkContext for those fast in-memory workloads.

This version is fully functional (see README.md - TL;DR do "project jobserver" "re-start", then curl away) but intended for gathering feedback from the Spark community only.  As such:
- There are no tests - we have a full suite of tests which we would be happy to push later
- Package names have not been adjusted to org.apache..... 
- Sorry, there are no examples.  We can push some examples soon.

A good way to get started is:
- read the README
- read the docs (there are some diagrams which you need to input into websequencediagrams.com)
- have a look at SparkJob.scala
- have a look at WebApi.scala