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)
Merge pull request #451 from melissalinkert/index16-color-model

Fix the component retrieval methods in Index16ColorModel