0.2.0
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- Add class methods to get last and next scheduled occurrence.
- Pass last and next (current) occurrence to #perform, if desired.
  This checks the method arity of #perform.
- Bump Sidekiq dependency to 2.8.0
- Fix incorrectly assigned Thread priority.
- Adjust clock sleep depending of execution time of the last tick.
- Don't log thread object ids.
- Issue a warning from the middleware if the clock thread died previously.

0.1.5
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- Allow jobs to be scheduled for immediate runs via the web extension.