Switch Async tests to importing mock module
Add unit tests for Async result persist logic
Make line length shorter
Add support for unique id to Async

If an id is given, the Async will use that id.  If an id is not given, a
unique id will be generated at instantiation time.
Make sure that passing id to update_options works as expected
Merge pull request #123 from WebFilings/async-id

Make Async id work right and test persist result