Add purge() to clear tasks from queues
Add all_queues_from_queue_service

Add pullqueue_names_from_queue_service().
Fix opposite names was returned for pullqueue_names_from_queue_service().
Add get_tasks() to retrieve tasks from queues
Add add_tasks()

This allows adding or readding of tasks to queues.

Useful with get_tasks() for manipulating which tasks should be run
during integration tests.
Add new functions add_task, etc. to __all__
Avoid calling Queue.add() with empty lists

In some cases add() gives an error if adding an empty list of tasks.
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
Merge pull request #89 from ericolson-wf/purge_get_add_tasks

Add purge(), get_tasks(), and add_tasks()
Merge branch 'master' into restart_v2
Add retry limit logic

Wire it up so that we use the built in app engine task retry limit. This
is set up in the to_task method such that a TaskRetryOptions object is
created and passed into the Task.

This also includes a few tweaks the the test stubs in how they handle
headers.