Add Ultrasonic Sensor HC-SR04

[garyanson]

There are already several libraries that support this sensor. One library that I find useful is NewPing.
Here is the bitbucket repo.
https://bitbucket.org/teckel12/arduino-new-ping/src/master/
I believe that integrating that library into the Adafruit_Sensor would be pretty straight forward.

[garyanson]

There is also a library in github
https://github.com/Martinsos/arduino-lib-hc-sr04