Allow plugins to register MQ Handlers

Hi Demis,
two things we love in SS: plugins to seperate our features and the MQ support. But there is a small issue combining those two:

We can register TypedRequestFilters through IAppHost in the plugins public void Register(IAppHost host) method. But there is no way to register a DTO with IMessageService.RegisterHandler<MyPluginDto>(host.ServiceController.Execute) because ServiceController doesn’t exist on IAppHost, “just” on ServiceStackHost.

Any reason why ServiceController isn’t on `IAppHost? Would be nice to have it there, so we don’t have to do the casts.

RegisterTypedRequestFilters came in here:

FYI, I’ve added ExecuteMessage and ServiceController on IAppHost in this commit.
This change will be available in the next release.

thank you! (post needs to be 20 chars)