SoftPWM v1.0.1

Fixes from PaulStoffregen:

• Use IntervalTimer on Teensy 3.x
• Use LED_BUILTIN for WLED

SoftPWM v1.0.0

Stable first version of the SoftPWM library.

This Arduino/Wiring library is strictly for AVR 8-bit microcontrollers, such as the ATmega328.