adding buildCSSUrls and buildJSUrls as low-level apis to build url and combo urls based on modules

* app.yui.buildJSUrls('dom', 'photos@hermes')
* app.yui.buildCSSUrls('cssbase', 'flickr@hermes')

This is useful if you want to do your own dependencies computations at the server side. For example, analyzing the logic pieces in your app and collecting a list of css modules that are required to be inserted at the header section of the page, in which case you can generate those urls using `buildCSSUrls` method.