Clones (#150)

* Provide property to Clone to distinguish "original" clones

* Provide method to clone a clone's properties

* Don't report clones in the UI target list

* Add target info to Thread

* Allow hats to skip clones (for green flag)

* Green flag skips clones

* Implement "create clone" and hat

* Pass the runtime to sprites and clones (for start hats)

* Clone disposal; trigger hats after drawable initializes.

* Separate stop threads for target; fix handling of stop button

* Remove extraneous `skipClones` property

* Add global clone limit

* Don't allow a non-clone to delete itself.

* Rename `cloneClone` -> `makeClone`

* Variable updates in runtime.js

* Synchronous drawable initialization (until we put it back to promises)

src/engine/execute.js

@@ -16,7 +16,7 @@ var isPromise = function (value) {
  */
 var execute = function (sequencer, thread) {
     var runtime = sequencer.runtime;
-    var target = runtime.targetForThread(thread);
+    var target = thread.target;
 
     // Current block to execute is the one on the top of the stack.
     var currentBlockId = thread.peekStack();