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.
I may have misunderstood it, but it's done on line 38 & 47.
I call `builder.setPrincipal(principal)` only if I have a `principal`.