SPARK-1230: [WIP] Enable SparkContext.addJars() to load classes not in CLASSPATH

Reopens and up-merges @velvia's patch previously against the old repo:
https://github.com/apache/incubator-spark/pull/299

In addition to merging I made some changes, one of which I'm not sure is correct. The change was to avoid setting the classloader explicitly in akka. I'm planning to add unit tests to this tomorrow to play around with the change.