Add normalize() for simple histogram stretching

Available as normalize() or normalise().
Normalization takes place in LAB place and thus should not change any
colors.

Existing alpha channels are preserved untouched by normalization.

test/fixtures/index.js

@@ -15,9 +15,12 @@ module.exports = {
   inputJpgWithCmykProfile: getPath('Channel_digital_image_CMYK_color.jpg'), // http://en.wikipedia.org/wiki/File:Channel_digital_image_CMYK_color.jpg
   inputJpgWithCmykNoProfile: getPath('Channel_digital_image_CMYK_color_no_profile.jpg'),
   inputJpgWithCorruptHeader: getPath('corrupt-header.jpg'),
+  inputJpgWithLowContrast: getPath('low-contrast.jpg'), // http://www.flickr.com/photos/grizdave/2569067123/
 
   inputPng: getPath('50020484-00001.png'), // http://c.searspartsdirect.com/lis_png/PLDM/50020484-00001.png
   inputPngWithTransparency: getPath('blackbug.png'), // public domain
+  inputPngWithGreyAlpha: getPath('grey-8bit-alpha.png'),
+  inputPngWithOneColor: getPath('2x2_fdcce6.png'),
 
   inputWebP: getPath('4.webp'), // http://www.gstatic.com/webp/gallery/4.webp
   inputTiff: getPath('G31D.TIF'), // http://www.fileformat.info/format/tiff/sample/e6c9a6e5253348f4aef6d17b534360ab/index.htm