atmosphere-redis plugin support and moved ScalatraBroadcaster into a trait

This is my first attempt, so hopefully it doesn't look too bad! My main motivation was just to get the atmosphere-redis plugin working. I don't know if the plugin is something that should stay out of the core but I still needed to make the ScalatraBroadcaster more flexible. That is why I moved it into a trait. 

If this is something that everyone would be interested in, I would be happy to clean it up more or do anything else desired.

I think this might mainly be for casualJim's review, as I was told the atmosphere support has mainly been his baby.

Any comments/suggestions would be appreciated!


Commit log:
Made ScalatraBroadcaster into a trait for custom broadcasters. Moved some logic into a DefaultScalatraBroadcaster that gets used by default. Additionally made a RedisScalatraBroadcaster that uses the atmosphere-redis plugin. Custom broadcasters can be used by changing the new scalatraBroadcasterClass member in the AtmosphereSupport trait.