Add getTimeout() call to ATParser. Add payload for RX callback in BufferedSerial.

[janjongboom]

Because the way the ringbuffer works it's not possible to get the underlying buffer in the RX IRQ callback because then ATParser will miss characters. By adding the character as a parameter to the RX callback you can get the data in the callback, and ATParser will not miss anything (because you can implement another independent buffer).

@geky @sarahmarshy

[janjongboom]

Ping @geky @sarahmarshy