Improve Teensy3 timing using timer, works on 3.0 & 3.1

Phil B predicted my Teensy3 code based on arbitrary constants might someday have trouble.  Sure enough, it broke with Teensy 3.1's faster flash memory.

This patch paces the waveform to a timer with parameters that properly scale with the clock frequency.  I really should have done it this way from the start.

All edits in this pull request are to the Teensy3 code only.  AVR code was not touched.