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")" />
~~~

Ok, I see this test here, but this isn't testing the reverse router, it's only testing the forward router.  The purpose of the PathBindable is for reverse routing - during forward routing, it's effectively a noop (note the implementation of the `PathBindable[String].bind` method below, it just returns the value as is).