ArduPPM V2.3.13
- New interrupt system that handles certain Futaba receivers better (simultaneous changes on groups of R/C channels in fast intervals)
- Improved active channel detection requering 100 valid pulses before channel is marked active

- Removed forced throttle fail-safe after channel loss
- Lost channel detection signal for APM by setting channel output to 800us (not activated yet, need APM code to handle signals)
Rover: set throttle range from -100 to 100

this allows for reverse
AP_HAL: Stub implementations of pure virtual AP_HAL classes
AP_HAL_AVR: Stub implementations of AP_HAL_AVR concrete classes
AP_HAL: import utility Print, Stream, and BetterStream headers
AP_Loop: moved to APO library, which is the only code that uses it
AP_Param: move from AP_Common to its own library
AP_Param: #include <AP_Param.h> fixups for libraries & sketches

* I mostly went through with grep and added an #include <AP_Param.h> below
  every #include <AP_Common.h>. Not all of these example sketches might
  strictly need AP_Param.
Remove empty AP_Var_menufuncs.cpp
AP_HAL: add BetterStream, Stream, and Print interfaces

* BetterStream Means AP_HAL depends on AP_Common, for now, in order to have
  the AVR specific pgm_char_t defined.
  I'll need to factor that out in the future but for now it can stay
AP_HAL_AVR move library classes to their own AP_HAL_AVR namespace
Arduino.mk: support nocore.inoflag to do coreless builds of each sketch
Arduino.mk: when core is excluded, add -DEXCLUDECORE to flags
AP_Common: use EXCLUDECORE macro to hack some debug messages away
AP_HAL_AVR: header include fixups
AP_HAL_AVR: build APM1 example sketch with no Arduio core via nocore.inoflag
AP_HAL: add virtual init method to HAL class
AP_HAL_AVR: implement ::init method in derived HAL_AVR class.

* Implementation of ::init taken from Arduino core. Not tested yet
AP_HAL: Add scheduler interface, move Arduino init code to implementation
AP_HAL_AVR: Blink sketch - first working code on coreless