Framework for extending Ace session state

[dabbler0]

Droplet needs a way for plugins that modify Ace behavior to add information to the serialized session state that Ace persists between refreshes. This adds the following public API method:

ace.extendSerializedState(
function serialize(aceSession) {
  return someObject; 
},
function deserialize(someObject, aceSession) {
  // take someObject stored by previous function and modify the session accordingly
}

This should also be useful for other plugins that want to add persistent functionality to the default Ace plugin.

@kzidane @dmalan