Max!CUL Binding - Handle TX credit via hardware

These changes implement the transmit credit system using the value retrieved from the CUL USB stick (on any RX or TX event). The implementation is such that it's up to the binding to implement the protocol specific queuing and holdoff required, the IO implementation simply provides a mechanism to report the current credit status.

I have also implemented a binding configuration that allows the user to monitor the credit status (to check for any bottleneck issues) from OpenHAB. Details of this have been added to the wiki entry for Max!CUL.

Note: Network implementation has been done, but it is not tested. It is assumed that the same command works but I don't have hardware to test on.