Asset versioning

ticket 842 https://play.lighthouseapp.com/projects/82401/tickets/842-proper-asset-versioning

When doing a reverse routing on an `Asset` we insert a hash of the content into the generated url.
So in a template : `@routes.Assets.versionedAt("javascripts/jquery-1.7.1.min.js")` will be transformed to `/assetsVersionned/javascripts/jquery-1.7.1.min-9eb9ac595e9b5544e2dc79fff7cd2d0b4b5ef71f.js`

todo : javascript reverse router