Added docs for the simple group management feature.
Fixed incorrect function name in example.
Added work_queue:specify_password_file() and updated docs to demonstrate it.
Merge pull request #7 from dthain/master
Adds work_queue_enable_monitoring function.

This function replaces work_queue_create_monitoring. Instead,
work_queue_enable_monitoring is called with an existing queue.
Adds sink/source interface functions for the dag.
Adds set data structure to dttools.
Explicit ancestor/descendant nodes in makeflow.