include worker id and host in log output

Makes log output more usable for troubleshooting when somewhere a worker process is hung. The "is currently run by worker-RANDOM_NUMBER" error message will now include the host, and logs on that host will let associate it with the pid.
Merge pull request #323 from spotify/ulzha/worker_ref

include worker id and host in log output
Hotfixed an error caused by c75085b3c812c8054ef76d39676ab473e45b9600
Merge branch 'master' of https://github.com/spotify/luigi into workerless-task

Conflicts:
	luigi/scheduler.py