consider asynchronous version and/or threaded pinging #3

Currently:

• the PID.send method blocks for the specified timeout value (0.5 seconds by default); and
• the user must manually manage ping-ing to prevent the display from blanking.

It would be nice if the PID.send method didn't block. An asynchronous version of this method could utilise futures instead.

It would also be nice if, whether through a thread or in an asynchronous environment, the library automatically ping-ed the display at some appropriate interval so that the end user doesn't need to worry about it.