Release Name: 0.2.0

Notes:
* Now pure Ruby
* Added the TaskScheduler#exists? method
* One bug fix
* Some doc cleanup


Changes:
* Rewritten in pure Ruby!
* The TaskScheduler::ONCE constant is now a valid trigger type. Thanks go to
  Uri Iurgel for the spot and patch.
* Added the TaskScheduler#exists? method.
* Added the TaskScheduler#tasks alias for the TaskScheduler#enum method.
* The TaskScheduler#new_work_item method now accepts symbols as well as
  strings for hash keys, and ignores case. Also, the keys are now validated.
* Renamed the example file and test file.
* Added the 'example' Rake task.
* Fixed some code in the README synopsis that was incorrect.