updated TagPubRF12toSerial sketch to be able to act as bi-directional

update TagExchange lib with support for Serial in/out

add new examples for testing the serial and ethernet TagExchange.

refactor classes; move duplicated logic to base class.

add class for stream objects; initial testing for sending over Serial and

improve handling of transmit buffer for RF12M. Dequeue immediately when

implement timeout handling for automatic dequeueing of the transmit buffer

move data definitions to base class; make the RF12M library a subclass.

Initial check-in of the TagExchange library... still a work in progress

Initial commit