Device specific stylesheets

Any sass files prefixed with the device type (ios, droid, web) will only be included on that particular device; also, those files will be included after all other style sheets.

The manifest will list all the css files in the shell, the app builders will then filter that list removing any stylesheets prefixed with device ids that aren't the current device. Those files are exposed to the views through a `style_files` helper.

I'm not sure if this is the best implementation, it feels a bit rough. Open to suggestions.

The main use-case for this right now is for removing certain aspects of pages on devices; for example, hiding the header when iOS is using a navigation bar.