Author: Tom Byrne
Driver class for a DHT22 temperature/humidity sensor.
Datasheets for this part are quite numerous and varied; this one was useful in writing this class.
The DHTxx family uses a proprietary 1-wire protocol. The imp can emulate this protocol with SPI, using both the MISO and MOSI lines. To use, tie MISO and MOSI together with a 1kΩ resistor, then tie MISO to the DATA line on the DHTxx.
