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.
From the FrameworkInfo.principal protobuf comment: (Lines 114-117)
* The principal field should match the credential the framework uses
* in authentication. This field is used for framework API rate
* exporting and limiting and should be set even if authentication is
* not enabled if these features are desired.