Splash screens, easy icon, and platform-specific preferences

This adds support, tests, and documentation for 
* the top-level `icon` element from which Cordova derives platform-compatible icons
* the `splash` element within each platform
* the `preference` element within each platform
as documented [here](http://cordova.apache.org/docs/en/4.0.0/config_ref_images.md.html). There is also some cleanup and documentation for the `icon` element within each platform that was added by another contributor in an earlier pull request.

Sorry for the big PR, but due to the interdependencies between these changes (especially in the tests), multiple pull requests would have been more difficult to merge.