Tidied up interrupt handler pointers. (This resulted in 8 _more_ bytes (4 x 32bit words) of binary code. :-/ Oh well. It's more consistent with the rest of the code, now.
Trivial: removed ambuigous,'Spark_' from USART_Interrupt_Handler(...)
Edited README.md to explain purpose and method of this branch.

Added ::end method code to undo pin re-mapping.