Add Abort functionality

Adding async.Abort, which is an exception that when thrown within a job
causes the job to immediately execute with only a logging message and no
callbacks executed.
Add clarifying statement to Abort docstr.

Add a statement to the Abort docstr indicating that only an info level
log message will be output upon Abortion.
In test_Abort, add checks on logging and start

In the test_Abort test, I added a check to ensure that Async.start() is
not called. Also, I added checks on logging.info() and logging.error()
to ensure that info is called and error is not.
Add logging intercept to test_Abort

Add a logging handler to the root logger that will throw an Exception
upon receiving a log message with an Error level or greater level.
Merge branch 'master' into add_abort
Merge pull request #48 from tannermiller-wf/add_abort

Add Abort functionality