Bug fix: Interrupt automaticaly deleted

Hi Notro,

A new commit for this module. I finally found and fixed it.

1st bug: Previous commit partially fixed bug. On module unloading, IRQ where not freed up.
2nd buig: with switch bouncing, shutdown function is called multiple times, trying to free IRQ multiple times (bad), without doing a proper shutdown.
Now, IRQ is freed up only in unload module function (much better)

All the best!

Martin
PS: as usual, I can advertise on this update on Adafruit forum