Premultiply alpha channel to avoid dark artifacts during tranformation

Add `Sharp.compare(file1, file2, callback)` function for comparing images
using mean squared error (MSE). This is useful for unit tests.

See:
- https://github.com/jcupitt/libvips/issues/291
- http://entropymine.com/imageworsener/resizealpha/

package.json

@@ -20,7 +20,10 @@
   ],
   "description": "High performance Node.js module to resize JPEG, PNG, WebP and TIFF images using the libvips library",
   "scripts": {
+    "clean": "rm -rf test/fixtures/output.*",
     "test": "VIPS_WARNING=0 node ./node_modules/istanbul/lib/cli.js cover ./node_modules/mocha/bin/_mocha -- --slow=5000 --timeout=20000 ./test/unit/*.js",
+    "test-clean": "npm run clean && npm install && npm test",
+    "test-leak": "cd test/leak; ./leak.sh; cd - > /dev/null",
     "test-win32-node": "node ./node_modules/mocha/bin/mocha --slow=5000 --timeout=20000 ./test/unit/*.js",
     "test-win32-iojs": "iojs ./node_modules/mocha/bin/mocha --slow=5000 --timeout=20000 ./test/unit/*.js"
   },