Add optional, automatic resizing to nodes

[JoshAshby]

As per issue #39 this gives nodes the ability to resize according to the GUILayout content inside of them. I made this optional and defaulted it to being off, since it'll probably break the size of old nodes.

To use, override Vector2 MinSize and bool Resizable and only set the rect.position:

        public override Vector2 MinSize { get { return new Vector2(200, 10); } }
        public override bool Resizable { get { return true; } }

        public override string GetID { get { return "resizingNode"; } }

        public override Node Create(Vector2 pos)
        {
            ResizingNode node = CreateInstance<ResizingNode>();

            node.rect.position = pos;
            node.name = "Resizing Node";

            return node;
        }

[Seneral]

Thank you! Really cool:) Turning it off by default was good, this would overwrite the width forever as size is only set in Create so far. But I have plans to make everything in Create set as parameters which would make it easily possible to turn it on by default...