Support Mesos Framework Authentication

This is a basic and naive implementation for #638.

I don't speak Scala, so this is not tested, and may be very wrong but I've successfully authenticated against my Mesos Master :)

Let me know what you think about that.
At this point, we should also set the `principal` value before building the `FrameworkInfo`.  See the optional value here: https://github.com/apache/mesos/blob/master/include/mesos/mesos.proto#L130.