Colorize raster bands

Since GDAL doesn't seem to support adding color tables to Float32 images (only Byte and UInt16), I had to add the ability to calculate NDVI using Floats, but still render the other types of images.  This accomplishes that.  Got some help from https://gist.github.com/celoyd/7443646 for the algorithm.

The RAT changes aren't related--just some bugs that I found along the way.