Update lib/crc.js
Update lib/crc.js

added modbus support

alexgorbatchev/node-crc,albertjleo/node-crc,alexgorbatchev/node-crc,alexgorbatchev/node-crc,alexgorbatchev/node-crc,IvanSanchez/node-crc,IvanSanchez/node-crc,IvanSanchez/node-crc,albertjleo/node-crc,alexgorbatchev/node-crc,IvanSanchez/node-crc,albertjleo/node-crc