ENH: Handshake protocol between hub and sensor

[jschrempp]

When the sensor sends a message it then waits for an acknowledgement from the Hub. If it doesn't get this, then it will retry sending the message two more times before giving up.

[BobGlicksman]

I think that we need to define some different sensor types and make separate folders for code for each. Then we can code and test these various features while not committing one specific feature set to github and losing other feature sets.

[BobGlicksman]

At least, we need a #define to determine if the sensor should process a response from the hub or not.