WIP: Native OS X image reader

This is still in progress, but is pretty useful at the moment.  I've tested with 8 and 16 bit grayscale tiffs, and an 8 bit png.  In principle, it should be able to pull in the most common image formats.  The nice thing is that we can guarantee that these libraries will be on any OS X system from the start.  It's also pretty snappy, loading a 250 MB tiff stack in under a second.

Before it's ready, it will probably need to handle floating point grayscale, as well as color images.  It will also need to gracefully switch over to ImageMagick when it fails, to avoid irritating folks.

Let me know if you have any suggestions.