Render start and stop callbacks for all cases.

Rendering can happen on demand (by calling start()) but also when adding/removing nodes and relations. I also need to be informed when rendering starts in my use case. I have a re-arrange button that needs to be toggled whenever rendering is in progress, and with these events it's nice and clean.
When the event callbacks are passed in to the Springy.Renderer start() method only (as in my previous commit) then all the other cases are left out (adding/removing nodes/relations).