Add more events, event feedback

[skeate]

Right now only the L.Timeline layer has an event, and it doesn't tell you anything (though you can access most of what you might want to know directly through the L.Timeline object).

Would be nice to have the slider control also trigger events, and pass data through the events as well (layer event could tell you the displayed shapes, control event could tell you all displayed shapes for all layers, e.g.)

From #23