Initial async work

Operations can now:
1) return their progress directly.
2) throw an error.
3) (ADDED) return a promise:
  + that resolves to their progress.
  + that rejects an error message (like throwing but asynchronous).
  For an example see the new operation "Wait" (Flow Control)

Added a flow control operation "Wait", which waits for the number of
milliseconds passed in as its argument. It is a fairly useless operation
but it does demonstrate how asynchronous operations now work.

A recipe like:
```
Fork
Wait (1000ms)
```
will only wait for 1000ms (each wait runs at the same time as each
other).

I have not looked into performance implications yet, also this code is
probably more complicated than it needs to be (would love help on this).

src/js/config/Categories.js

@@ -285,6 +285,7 @@ var Categories = [
             "Jump",
             "Conditional Jump",
             "Return",
+            "Wait",
         ]
     },
 ];