Abstract nativeclient callback logic

The nativeclient.js module overrides
window.textsecure.registerOnLoadFunction with its own version. Otherwise
helpers will define a trivial placeholder for same.

The flag textsecure.NATIVE_CLIENT can be set anywhere ahead of
nativeclient.js, but is only acted on in nativeclient.js,
and crypto.js.

js/nativeclient.js

@@ -16,6 +16,11 @@
 ;(function() {
     'use strict';
     window.textsecure = window.textsecure || {};
+    window.textsecure.NATIVE_CLIENT = window.textsecure.NATIVE_CLIENT || true;
+
+    if (!textsecure.NATIVE_CLIENT) {
+        window.textsecure.registerOnLoadFunction = window.textsecure.nativeclient.registerOnLoadFunction;
+    }
 
     var naclMessageNextId = 0;
     var naclMessageIdCallbackMap = {};