Reverse Router should use minified assets in production automatically

Serving assets is usually different between development and production mode. In dev mode, you want to serve the assets as they are, and in production they should be minified, compressed, versioned, and so on.

Using minified resources already works with RequireJS assets that go through r.js. Using versioned assets in prod mode works thanks to the Router's `versioned` method. Now it'd be nice to combine this, so Play serves versioned and minified LESS/CSS assets.

To put it in code:

~~~HTML
@if(current.mode == Mode.Dev) {
  <link rel="stylesheet" media="screen" href="@routes.Assets.versioned("lib/bootstrap/css/bootstrap.css")" />
}
@if(current.mode == Mode.Prod) {
  <link rel="stylesheet" media="screen" href="@routes.Assets.versioned("lib/bootstrap/css/bootstrap.min.css")" />
}
~~~

should become

~~~HTML
<link rel="stylesheet" media="screen" href="@routes.Assets.versionedAndModeAware("lib/bootstrap/css/bootstrap.css")" />
~~~

Well... we've got to start somewhere... :)  I'll merge it, but only if you add a comment to this PR with a picture of a kitten in it.  I like kittens.