Expose aplication ID in ApplicationStart event, use it in history server.

This change exposes the application ID generated by the Spark Master, Mesos or Yarn
via the SparkListenerApplicationStart event. It then uses that information to expose the
application via its ID in the history server, instead of using the internal directory name
generated by the event logger as an application id. This allows someone who knows
the application ID to easily figure out the URL for the application's entry in the HS, aside
from looking better.

In Yarn mode, this is used to generate a direct link from the RM application list to the
Spark history server entry (thus providing a fix for SPARK-2150).

Note this sort of assumes that the different managers will generate app ids that are
sufficiently different from each other that clashes will not occur.
So, what happened is that when I wrote this code there was no such thing as YarnClusterScheduler. So I created it, and moved its initialization to the backend class. Later someone else added the same class and I just merged my code with theirs.

I think this is cleaner because it removes reflection code from SparkContext. Unless you think that someone will ever try to match YarnClusterSchedulerBackend with something other than YarnClusterScheduler.