Track the number of insert success and failures on Context.

This adds two new properties to Context which tracks the
number of tasks that were successfully inserted and tasks
that failed to be inserted.
Update context unit tests for insert_tasks changes.
Clarify comment in _handle_tasks.
Add unit test to ensure counters are calculated correctly.
Merge pull request #103 from tylertreat-wf/insert_counter

Track inserted/not-inserted tasks on Context