Small fixes for the Spindle module.

- Now uses us_ticker_read(), leaving systick free for other uses.
- Sets lower priority for the pulse sensor interrupt.
- Added option for smoothing filter on the RPM readings, which
  helps PID if the readings are noisy.
Merge pull request #574 from PetteriAimonen/spindle_fixes_for_merge

Small fixes for the Spindle module.