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.
> It would be safer to read the authentication secret from a file. Do you feel comfortable making this change?

I'll try to find how to open & read a file.
Like I said, I didn't read or write any scala before this afternoon :)