Lots of changes here. Added new I2C test code, a new serialTest program,

Lots of changes here. Added new I2C test code, a new serialTest program,
and developed the new ISR - Interrupt Service Routine
handler - much easier than the old waitForInterrupt code!

Minor tweaks to the gpio program to recognise the environment variable
WIRINGPI_DEBUG too, and removed the printing of the errors from the
main wiringPi setup routines (and added some new ones!)