Updated JsonpFormatter for WebAPI release version

This is an updated version of JsonpFormatter.cs based on the released version of WebAPI.

See [this thread on StackOverflow](http://stackoverflow.com/questions/9421312/jsonp-with-asp-net-web-api), post by Peter Moberg.  I had to change the HttpContentHeaders to HttpContent to get it to work. 

Thanks for writing this useful class, much appreciated.