Changes to debugging, battery alarm, ppm timing:

Changes to debugging, battery alarm, ppm timing:

* Added a #define DEBUG statement, and a bunch of #ifdef/#else/#endif that includes or excludes debugging code depending on this #define
* Added BATTERY_CRITICAL, and changed the way BATTERY_LOW was handled. We now have two states of "severity" on the battery
* Set the debugging-value of BATTERY_CRITICAL to 0, to be able to debug without a battery connected :P
* Changed PPM timings again, had problems with some RF recievers. Tested OK with: fmsusb2, Graupner RX700, Corona RS810II, Corona RP8D1.