Fix the component retrieval methods in Index16ColorModel

Byte-swapping the values is not correct, and getRed(int), getGreen(int),
and getBlue(int) should normalize the return values to 0-255, as per the
Javadoc for ColorModel:

http://docs.oracle.com/javase/1.5.0/docs/api/java/awt/image/ColorModel.html#getRed(int)

See http://lists.openmicroscopy.org.uk/pipermail/ome-users/2013-April/003654.html