Interrupt driven DHT 11/21/22 sensor library for Spark

Supports reading DHT22/DHT21/DHT11 sensors without disabling interrupts

Scott Piette, Piette Technologies - scott.piette@gmail.com
Jan 18, 2014  - Initial port to Spark Core
Oct 01, 2014  - Added DHT11/21 support, fixed timing issues, enhanced examples
Sep 23, 2016  - Updated for Particle and moved isrCallback

VERSION:	0.4
PURPOSE:	Interrupt driven non-blocking library for DHT sensors
LICENSE:	GPL v3 (http://www.gnu.org/licenses/gpl.html)

BASED ON:

DHT11 interrupt library https://github.com/niesteszeck
DHT11 library http://playground.arduino.cc/Main/DHT11Lib

Wiring diagram for Spark Core with DHT22 Sensor

DATASHEETS:

DHT11	http://akizukidenshi.com/download/ds/aosong/dht11.pdf
DHT22	https://www.sparkfun.com/datasheets/Sensors/Temperature/DHT22.pdf