Allow attributes to be backed by methods, not just plain attr_accessors

Hey there,

So I've tweaked the dynamic mbean class a bit so that you can do things like:

```
class HttpServerMBean < JMX::DynamicMBean

  def initialize(http_server)
    @http_server = http_server
  end

  r_attribute :thread_count, :int, 'number of threads'

  def thread_count
    @http_server.thread_count
  end
end
```

It didn't work as it was, because it was fetching values by accessing the instance variable directly, which didn't seem necessary.  I could have gone a lot further, you can see the code above code benefit from some ruby meta magic, but this was just enough to get in what I needed.  I also made another small change so you can connect via pid, useful when you have a pidfile somewhere, as I often do.


Cheers,
Nick