Fixed bugs in gwf-clean
Removed gwf-print-targets.

gwf -v will show the same relevant output.
Updated setup.py
* Changed graph.schedule, to accept a list of targets
* Added a graph.tasklist to return all tasks from a list of targets
* Added output of "running and skipped" tasks to gwf
* Changed the use of 'sets' to 'lists' in dependency_graph.py such that task lists are ordered in an optimal fashion, to reduce the amount of tests by the TaskScheduler