NewPing Library (Ultrasonic Sensors)
Written by Tim Eckel
https://code.google.com/p/arduino-new-ping/
Modified for Teensy 3.0 & 3.1
https://github.com/PaulStoffregen/NewPing
http://forum.pjrc.com/threads/25907-Multiple-HCSR-04-Ultrasonic-sensors-on-teensy-3
