Replaced icon() with a more complete method icons()

It returns a hash with an item for every icon found. The key is the full name (e.g. Icon@2x.png), the value is a proc returning the binary content of the file.

Beware that the PNG format used by Apple in these file isn't standard. Here is an example of how to convert it to standard PNG: https://gist.github.com/AquaGeek/4693925.