Implementation of Actor DSL

See #355

Actor DSL gives possibility to establish new actor behaviors on demand, using configurable lambda function instead of creating new actor class.

Example usage:
```
var act = system.ActorOf(config => {
    config.OnPostStop = context => context.Sender.Tell("stopping");
    config.Receive<string>(msg => msg == "ping", (msg, context) => context.Sender.Tell("pong"));
});
```