#192 fixes the infinite wait after a forever pending node
cluster: minor cleanup to wait_for_running_instances

* Make the timeout for terminating a pending instance a configurable
  kwarg: kill_pending_after_mins
* Log a warning when terminating instances that have been pending for
  too long.
Merge branch 'fix-forever-pending-nodes' into develop

Conflicts:
	starcluster/cluster.py

closes gh-192
closes gh-211