support for registration 2 response (JSON)

response is simple string formatted as JSON structure, see MSP430 example
MSP430 example: support for JSON registration response
PIC16F88 example: support for JSON registration response
mark all headers with "C" external linkage

now it's possible to use 'uC' framework on Arduino devices: just copy all files from 'include' and 'src' directories to the ${arduino}/libraries/DeviceHiveEngine directory.
merge JSON registration response feature