XBeeBase constructor should accept an optional callback function
argument. When provided, this will put the module into a threaded
mode, in which it will call the provided function with any API
frame data received.
As it would be very difficult to sanely test an asynchonous callback
routine with a synchronous test process, proper callback behavior
is not tested automatically at this time. Theoretically, the
callback implementation logic is simple, but use it at your own risk.
Definition at line 138 of file test_base.py.