Correct directory in log message "INFO: Logging events to <dir>"
Master UI - Rebuild SparkUI on application finish

The Master UI now links to a reconstructed SparkUI when an application finishes,
instead of keeping around a broken link. This involves exposing the path to the
directory used for event logging.

This commit also allows us to get rid of the SparkListenerApplicationStart event,
which contains only the application name. Further, the app URL is a duplicated
parameter in ApplicationInfo and ApplicationDescription, and the app name is
passed as an extraneous argument to creating the backend schedulers in SparkContext.
These are both fixed.

TODO: Master currently does not know how to read compressed event logs.