added RouteDataMapping MediaTypeMapping

Implemented `RouteDataMapping` `MediaTypeMapping`.

Sample usage:

Add a route which satisfies our needs:

    GlobalConfiguration.Configuration.Routes.MapHttpRoute(
        "defaultHttpRoute",
        routeTemplate: "api/{controller}.{extension}",
        defaults: new { },
        constraints: new { extension = "json|xml" }
    );

Add the `RouteDataMapping` instance to formatters:

    GlobalConfiguration.Configuration.Formatters.JsonFormatter.
        MediaTypeMappings.Add(
            new RouteDataMapping(
                "extension", "json", "application/json"
        )
    );

    GlobalConfiguration.Configuration.Formatters.XmlFormatter.
        MediaTypeMappings.Add(
            new RouteDataMapping(
                "extension", "xml", "application/xml"
        )
    );

For more information on `RouteDataMapping`:

[ASP.NET Web API Custom RouteDataMapping (MediaTypeMapping)](http://www.tugberkugurlu.com/archive/asp-net-web-api-custom-routedatamapping-mediatypemapping)