Library
https://github.com/adafruit/Adafruit-Fingerprint-Sensor-Library
Tutorial
https://cdn-learn.adafruit.com/downloads/pdf/adafruit-optical-fingerprint-sensor.pdf
https://github.com/adafruit/Adafruit-Fingerprint-Sensor-Library/blob/master/documentation/ZFM-20_Fingerprint_Module.pdf
http://www.instructables.com/id/Arduino-Fingerprint-Lock/
http://blog.sina.com.cn/s/blog_54bf41d30102w33v.html
