Allow enabling/disabling external interrupts and clear pending interrupts on attach

This is a fix for #510, plus the addition of a new `enableInterrupt` and `disableInterrupt` API to cater for libraries that relied on the broken behaviour of `attachInterrupt`.

See also this discussion: https://groups.google.com/a/arduino.cc/forum/#!topic/developers/3n_XJnelPzM

This code is still unfinished:
 - Only quickly compile tested
 - Not implemented on SAM yet