Get rid of the Command class completely.

GoalRunner now just extends object.

The functionality previously in Command is now in GoalRunner.

Most of the functionality in pants_exe.py is now in GoalRunner too.

RIP old pants...

Testing Done:
All integration and unit tests pass.

Reviewed at https://rbcommons.com/s/twitter/r/1465/
Sync of https://github.com/pantsbuild/pants/commit/6a30e22fa06ec2801136bb59d5cc8b639e8e7394