Add async task insertion to Async

If you pass async=True into Async.start() then it will use the
asynchronous add method of the Queue instead of the regular one.
Fix a few notes

Move an import and re-add a TODO note.
Merge pull request #107 from tannermiller-wf/async_task_insertion

Add async task insertion to Async
Ensure task batches are not greater than 100

When using AutoContext, a custom batch_size can be set.
Force the custom batch_size to always be less than 100 since
that is the max that can be added to an app engine taskqueue at a time.
Merge pull request #113 from ericolson-wf/100tasks

Ensure task batches are not greater than 100