Ugh, pry
Add ring buffer to store recent values in plugins

- Add new dummy 'sin(t)' plugin
- Update docs
- Convert existing plugins to use new method
expect floats in ScalerPlugin interval/hysteresis_period
Fix tests to handle hysteresis
Add test for error handler
Merge pull request #3 from harrystech/relaxation_logic

Modify 'hysteresis' behaviour