Adds engine.filter method for getting renderables

Pass the Factory returned by glitz.Renderable to `filter` to
retrieve a set of all instances of that factory regardless of heirarchy.

    var Box = glitz.Renderable({ ... });
    engine.push( new Box({ ... }) );
    engine.filter( Box ); // => [ box, box, box, ... ]

Optionally pass a specific renderable to filter only from it's children.

    engine.filter( Box, engine.layout ); // default behavior