Fastjpeg

stb_image: NEON kernels for JPEG decoder.

This adds IDCT, resample hv2, YCbCr->RGB exactly matching the SSE2 (and updated integer)
counterparts and updates the docs. Using these kernels makes decoding anemones.jpg about
1.5x faster on my Nexus 10.

I removed the resampler code from the "JPEG_OLD" path since the SSE2 resampler and the
regular path should match exactly - if they don't, that's a bug. All of these kernels have been
subjected to randomized testing against the C versions on the target device, so I'm reasonably
certain they all match the intended behavior.

I think this code should be a bit more of a win than it actually is, but I haven't dug down into the
ASM to see what (if anything) is going wrong. In any case, this seems good enough to ship.