Only make the SchedulerDriver available after registered() has been called.

I've also removed some functions from the Driver interface, since they should
be used exclusively by SchedulerLifecycle.

Bugs closed: AURORA-45

Reviewed at https://reviews.apache.org/r/16995/
Improve test coverage for TaskAssignerImpl.

Bugs closed: AURORA-63

Reviewed at https://reviews.apache.org/r/17133/
Improve test coverage for CronJobManager.

Bugs closed: AURORA-62

Reviewed at https://reviews.apache.org/r/17131/
AURORA-75: Update all dependencies coming from twitter commons and upgrade to guava 15.

Bugs closed: AURORA-75

Reviewed at https://reviews.apache.org/r/17161/