Update to Arduino 1.0.5 (and for OSX avr-gcc 4.8.1)

This re-applies (and fixes) the ardupilot patches to Arduino onto the current upstream 1.0.5-r2-gitlatest version from upstream.  Also adds the MegaPilot-NG board definition.

For OSX only (so far) it also updates to use the avr-gcc 4.8.1 picked up from upstream Arduino's ide-1.5.x-avr-toolchain-gcc-4.8.1 branch

avr-gcc 4.8.1 compiles ardupilot to about 20% smaller size than the old compiler, and benchmarks on other codebases show a 1.5x-8x speedup at runtime (at the expense of slightly larger runtime RAM consumption).  This is a significant improvement that should be carried into the Windows and Linux builds as well, I just haven't got around to that yet since I don't work on those platforms.  It should be quite easy using the pre-built avr-gcc toolchains available from http://downloads.arduino.cc/avr-toolchain-${file_arch}-gcc-4.8.1.zip by modifying build/build.xml