This adds Radiance HDR image file support. So now at least we can save
the 'fbuf' (4x32 bits float colors) in Blender.
It doesn't change anything for internal support in imbuf for floa colors,
so when reading .hdr files it still converts it to 32 bits RGBA.
As an extra I've added that saving images with F3 now also adds the
optional extension, when the F10 "Extensions" option is set.
One important note; I don't know the proper license for the code, it was
provided without... will await feedback from Alfredo about it. For now
I've added the standard Blender GPL header.
