BitmapData.threshold() for CPP target.

BitmapData.threshold() implementation for cpp target. Pretty fast! Also tested and accurate - same visuals and return values as Flash API version. Got lots of help from crazysam (Sam Batista) and the HaxeFlixel community on this one.

This is my first pull request, so let me know if I did it wrong or need to fix something!

More details:
http://www.haxeflixel.com/forum/general-discussion/bitmapdatathreshold-palette-swap-solution-cpp-target
http://www.nme.io/community/forums/feature-requests/my-implementation-bitmapdatathreshold-cpp-target/#ccm-discussion-post-anchor

Three test images are used below: one grid of greyscale values, one grid of bluescale values, and one giant huge benchmark image full of yellow smiley faces (the benchmark color-swaps the yellow shades to cyan).

![nme_threshold_flash](https://f.cloud.github.com/assets/739064/353963/63400df8-a090-11e2-807b-c01db32ab29d.png)
![nme_threshold_cpp](https://f.cloud.github.com/assets/739064/353964/65529494-a090-11e2-8f0e-e8997b246ece.png)