Added I2C support to mbed-rpc
Hello, I've been adding I2C support to the official mbed-rpc library. It's basically just a copy paste action of the generic digital out rpc handler. Next I added method callers for all I2C class methods. Also, the arguments.cpp is updated to accomodate for some specific I2C class methods which had no matching template. The implementation has been verified on actual hardware using a mbed board (LPC1768) and some I2C expanders.