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.
Please refactor.  We could define `val credential: Option[Credential] = ...`.  This way `newDriver` can remain a `val`.

```scala
val newDriver: MesosSchedulerDriver = credential match {
  case Some(cred) => // ...
  case None       => // ...
}
```