Add an Exclude metadata property to Nodes.

Hi Richard,

I had a situation where this would be handy, it allows a node to have:

[Exclude]
var undesiredComponent : ComponentClass;

inside a Node definition, I've found it gives me some nice flexibility when designing Systems.

I have implemented it in a way that does not increase the default runtime performance. The only time any new code will run is if an excluded component exists.