Add support for Teensy 2.0 & 3.0

Small edits to add support for Teensy 2.0 (interrupt pin assignments) and Teensy 3.0 (uses IntervalTimer instead of AVR timer0).

Confirmed working by an Adafruit customer:

http://forum.pjrc.com/threads/23970-Teensy-3-0-Ada-FRuit-VS1053-Audio-Breakout-Board-Problems?p=34375&viewfull=1#post34375