Reduce threads created by Metrognome.

Moved the logic for thread management into Registrar and removed
Runner. We now only create a single thread per Scheduler, rather than
one per iteration.

We also get rid of the brittle file locking mechanism.
Add tests for Metrognome::Scheduler.

Test the scheduler class. Registrar is harder to test since the
interesting case is process exit after SIGTERM.

	modified:   .gitignore
	modified:   lib/metrognome.rb
	modified:   lib/metrognome/registrar.rb
	modified:   lib/metrognome/scheduler.rb
	modified:   lib/tasks/metrognome.rake
	new file:   spec/metrognome/scheduler_spec.rb
	new file:   spec/spec_helper.rb