API - Allow AST to be modified by more than one client

In Scratch we allow both the "player" and the blocks themselves to modify the state of a program. Because of this, we need a way to allow more than one (in our case: two) client to modify the representation of a program. Support for this is encapsulated in @NeilFraser 's "realtime" work. Placing here for further discussion and to track progress.

/cc @cwillisf @tmickel @rschamp 
