Update tests

Add test for playbook
Various cleanup to how tests to make sure things work as intended.
Add get_task_stats() to model.py
Add --stats option to ansible-report

This allows one to summarize just the stats of a query to the database.
Changes include:
* Adds a DEFAULT_STATS to constants.py
* Adds format_stats() to utils.py
* Updates output plugins screen and email to support stats summary
add comment to what playbook_on_notify does
Add changed column to task table