implement Script.buildMultisigOut()

lib/script.js

@@ -429,9 +429,15 @@ Script.prototype._addBuffer = function(buf, prepend) {
  * requiring m of those public keys to spend
  */
 Script.buildMultisigOut = function(pubkeys, m) {
-  console.log(pubkeys);
-  console.log(m);
-  return new Script();
+  var s = new Script();
+  s.add(Opcode.smallInt(m));
+  for (var i = 0; i<pubkeys.length; i++) {
+    var pubkey = pubkeys[i];
+    s.add(pubkey.toBuffer());
+  }
+  s.add(Opcode.smallInt(pubkeys.length));
+  s.add(Opcode('OP_CHECKMULTISIG'));
+  return s;
 };
 
 /**