Merge pull request #250 from FrankBoesing/patch-9

end() never disables interrupts

teensy3/IntervalTimer.cpp

 void IntervalTimer::end() {
 	if (channel) {
 		int index = channel - KINETISK_PIT_CHANNELS;
+		NVIC_DISABLE_IRQ(IRQ_PIT_CH0 + index);
 		funct_table[index] = dummy_funct;
 		channel->TCTRL = 0;
 #if defined(KINETISL)