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")" />
~~~

I didn't notice this one before.  Since it's on the Assets companion object, it never gets used unless you add it to the router imports.  And if you did do that, any reverse route that accepted a String path parameter (eg `/user/:username`) will use it, and will throw an exception from the code above (with message "Asset path bindable must be used in combination with an action that accepts a path parameter").  I think this should therefore be deleted, I don't think there's any purpose for it.