Shorten get_queue_names related functions

Now named:
 get_queue_names()
 get_pull_queue_names()
 get_push_queue_names()
Rename purge() to purge_tasks()

This is more consistent with its similar functions get_tasks() and
add_tasks().
Merge pull request #2 from ericolson-wf/purge_get_add_name_changes

Purge get add name changes