Add Play Asset JavaScript litteral binder

Without the proposed binder, Play doesn't compile the project with the message:

```
No JavaScript litteral binder found for type controllers.Assets.Asset. Try to implement an implicit JavascriptLitteral for this type.
```

when using the following route:
```
GET     /$file<(css|img|js)/.*>     controllers.CustomAssets.at(path="/public", file: Asset)
GET     /robots.txt                 controllers.CustomAssets.at(path="/public", file: Asset ="robots.txt")
GET     /favicon.ico                controllers.CustomAssets.at(path="/public", file: Asset ="img/favicon.ico")
```
It's the same as `litteralString`.