Wrap banner generation in a requirejs wrapper, for client side use

This a step towards having more common code between the client side building in `modernizr-ui` and server side building with `grunt`. It simply creates a function to generate a modernizr banner, given the modernizr version information

This can be used for client side generation by simply loading the `package.json` file with ajax, using requirejs to load the banner file (something like `require(['shared/generate', 'shared/banner'], function( generate, bannerGenerator ) {`), and adding `var bannerText = banner('compact', pkg);` to get the banner text.

Was working last night on some of the tasks in https://github.com/Modernizr/Modernizr/pull/1184#issuecomment-48089972, and this PR is a piece of the puzzle to item 2 under "Changes".