Add a JSONP Filter

This pull request adds a `Jsonp` filter in the `play-filters-helpers` project. This filter transforms JSON results into JavaScript results made of the JSON result preceeded by a padding, if the request query string contains a parameter providing the padding to use.

Note: I don’t handle `ChunkedResult`, I found no way to handle them. By the way, do we really need this type? It looks like everything could be expressed using `SimpleResult`.