Merge pull request #656 from hhzl/master

Update _st.js

Merging myself (it does not make any functionality change anyway), I want to slightly refactor this so I want it in.

support/_st.js

@@ -1,3 +1,15 @@
+
+/* 
+
+_st points to a function used all over the compiled amber code that
+takes any value (JavaScript or Smalltalk) and returns a proper Amber Smalltalk receiver.
+
+null or undefined -> nil,
+plain JS object -> wrapped JS object,
+otherwise unchanged
+
+*/
+
 define("amber_vm/_st", ["./boot"], function (boot) {
     return boot._st;
 });