Make EventSource’s Enumeratee return Events instead of Strings

We currently have to explicitly set the `Content-Type` of server-sent events responses. See e.g. [here](https://github.com/typesafehub/play-mongo-knockout/blob/master/app/controllers/MainController.scala#L30), [here](https://github.com/matthiasn/sse-chat/blob/master/app/controllers/ChatApplication.scala#L42) or [here](http://stackoverflow.com/questions/21766755/server-sent-events-with-angularjs-and-play-framework-keeps-restarting-after-su).

This PR automatically sets the right content type.