Add support for flushing and deleting tasks.

Clean up critic issues and fix optional serial param.
Add support for flush_all_tasks and delete_all_tasks to scheduler.
Add support for deleting tasks from the queues to taskqueue.
Update the Changes file.
Update version number in dist.ini.
Drop the ability to do Module::Build in dist.ini.
Add taskqueuectl support for flushing scheduled tasks.
Add taskqueuectl support for deleting unprocessed tasks.
Update tests to match new code.
On any tests that were changed replace hokey exception testing with
Test::Exception calls.

Based on discussion with Brian and Todd, I've added the flushing support. We had also talked a little about just being able to delete tasks instead of flushing them to be processed. Assuming that the flushing approach might not cover everything, I implemented both pieces of functionality. This prevents us from needed to make another release if the first idea is not enough.