Allow dynamic setting of message received handler

[256dpi]

As proposed by @sandeepmistry:

[..], however the Wire and new I2S Arduino libraries have a function to called onReceive to set a function handler for received data (https://www.arduino.cc/en/Reference/WireOnReceive). It would be nice to have this, instead of forcing sketched to define void messageReceived(...).

[tigoe]

I like this idea. I would also suggest that the two extra parameters, (char * bytes, unsigned int length) be given defaults, since they're not used in the example sketches. Alternately maybe use an overloaded function signature?

If char * bytes is absolutely necessary, it'd be nice to make it a little more in line with the Arduino API style guide (https://www.arduino.cc/en/Reference/APIStyleGuide). I get around it by doing char bytes[].

I'm glad this is already on the milestone list. Thank you for making a very nice library!

[256dpi]

This is beeing addressed in #59.