Add Image.normalize() support

My first attempt at adding functionality to Wand.  This pull request adds support for the [MagickNormalizeImage*](http://www.imagemagick.org/api/magick-image.php#MagickNormalizeImage) methods.

Edit: I've provided a very simple unit test that normalizes a grayscale gradient image and then checks for correct colors at the four corners.

Comments welcome.