Add experimental overlayWith API

Composites an overlay image with alpha channel into the input image (which
must have alpha channel) using ‘over’ alpha compositing blend mode. This API
requires both images to have the same dimensions.

References:
- http://en.wikipedia.org/wiki/Alpha_compositing#Alpha_blending
- https://github.com/jcupitt/ruby-vips/issues/28#issuecomment-9014826

See #97.

.jshintrc

@@ -4,6 +4,7 @@
   "maxparams": 4,
   "maxcomplexity": 13,
   "globals": {
+    "before": true,
     "describe": true,
     "it": true
   }