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.
A little confusing that AuthPrincipal is the principal string itself, but AuthSecret is a file. Maybe rename AuthSecret (and mesos_auth_secret) to AuthSecretFile/mesos_auth_secret_file? Or AuthSecretPath?
Also, it may be worth spelling out Auth as "Authentication", to distinguish it from "Authorization".