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.
One issue here is that the secret is an optional field in the Mesos protobuf message.  We shouldn't lose that flexibility here.  So, basically we can simplify the conditional expression guarding this block, and then If the secret flag was not supplied, we don't need to call `setSecret`.