I've implemented 	modbus client request/responses 1-6,15,16

The device I'm currently working with only supports functions 1,2,5,15,16, so the others are untested. The code should be the other functions should be very similar, but still... untested. The device also only has a single coil to test with.

Thanks for your previous work. Hopefully you can push this to the npm repository. :)

-Braden