Add endpoint for cancelling a build

- Adds a new BuildStatus: CANCELLED
- Adds an endpoint for changing the state of a build.  Currently only
the status can be changed and it can only be changed to CANCELLED.
- When a build is cancelled, we deplete any unstarted subjobs, then
call teardown on each slave.
- In the teardown call on a slave, wait for all executors to be idle
before posting idle status to the master.  This guards against a race
condition where a new build is submitted to the slave before all
executors are marked idle
- Client will cancel a build it has started if it is Ctrl-C'ed
Argh, these are starting to get pretty confusing -- I've stared at `is_finished` for awhile now and I'm still not sure  I'm completely understanding what it means. I wonder if there's a way we could simplify this, or if we're just at the point where we need to refactor this as something like a FSM. (Not suggesting you do that in this commit.)