Cleaned up ESP32 integration, reverted ESP32 ifdefs on irreceive examples.

- fixed indenting on existing code in a few places for consistency
- introduced IR_TIMER_USE_ESP32 for ifdefs within the code as per
  request
- added comments explaining what's missing for irsend support on ESP32
- IRrecvDemo.ino gets a warning before and after interrupt is enabled in
  case it causes a crash

TESTED=IoTuz ESP32 board and original 328p arduino to make sure current
code did not break.

irRecv.cpp

@@ -124,6 +124,8 @@ void  IRrecv::enableIRIn ( )
 {
 // Interrupt Service Routine - Fires every 50uS
 #ifdef ESP32
+	// ESP32 has a proper API to setup timers, no weird chip macros needed
+	// simply call the readable API versions :)
 	// 3 timers, choose #1, 80 divider nanosecond precision, 1 to count up
 	timer = timerBegin(1, 80, 1);
 	timerAttachInterrupt(timer, &IRTimer, 1);