Spark security part1

This pull request contains adding security to the following components. Other components will come in another PR.

- Akka 
- http server
- http broadcast
- web ui - added filters - yarn requires a particular filter (AmIpFilter) to be used to properly secure the UI. This makes it go through the ResourceManager web proxy. The web proxy has the ability to have filters, which one of them can be an authentication filter so you can use whatever normal auth you normally use.  
This was unfortunately a bigger change that I wanted as I had to switch to use servlets.  
People using spark standalone can install their own auth filter or any other type of filter.

Security defaults to off. For spark on yarn it handles generating a secret for you.  If you deploy a standalone cluster you have to set the SPARK_SECRET environment variable on the nodes.