Properly account for killed tasks.

The TaskState class's isFinished() method didn't return true for
KILLED tasks, which means some resources are never reclaimed
for tasks that are killed. This also made it inconsistent with the
isFinished() method used by CoarseMesosSchedulerBackend.
Reuse the set of failed states rather than creating a new object each time
Print run command to stderr rather than stdout
Merge pull request #818 from kayousterhout/killed_fix

Properly account for killed tasks.
Merge pull request #821 from pwendell/print-launch-command

Print run command to stderr rather than stdout