Support reinterpret(Uint8, img::Image{RGB{Ufixed8}}

See https://groups.google.com/d/msg/julia-users/-xxkIfv4IQQ/EpPTkJrbY8YJ

This requires that we do a little more runtime work, but overall
this code is easier to understand.
Implement fft & ifft for images (fixes #194)
Allowed imread to read 2-dimensional images.
added support for float16 encoded Imagine images

when mapping images to the RGB colorspace, NaN and Inf values are now set to zero.  The previous behavior caused crashing.
unfixed something that Tim already fixed recently