Added support for ResponseType Attribute on Controller Actions

As per http://www.asp.net/web-api/overview/web-api-routing-and-actions/action-results, the recommended way to return an Action response with a status code other than 200 is to set the return type of the action to `HttpResponseMessage`.
You can then either return an `HttpResponseMessage` directly or use the `Request.CreateResponse` method if you want to include the object.

As an example:
```csharp
[ResponseType(typeof(User))]
public HttpResponseMessage GetUser(HttpRequestMessage request, int userId, DateTime lastModifiedAtClient)
{
    var user = new DataEntities().Users.First(p => p.Id == userId);
    if (user.LastModified <= lastModifiedAtClient)
    {
         return new HttpResponseMessage(HttpStatusCode.NotModified);
    }
    return Request.CreateResponse(HttpStatusCode.OK, user);
}
```

The current version of WebApiProxy will set the return type of this action to `HttpResponseMessage` even though what we really want is `User`.

This change allows for the type specified in ResponseType to be used (as per the Web API Help Pages).