Expose current Builder instance as context properties System and `S…

…ystemJS`

That way systemjs-builder stays compatible with older configuration fiiles where the global `SystemJS` was used.

lib/builder.js

@@ -282,7 +282,7 @@ function executeConfigFile(saveForReset, ignoreBaseURL, configPath, source) {
   // only substitute local copy of System
   // and prevent that code from adding anything to the real global 
   var context = Object.create(global);
-  context.System = configLoader;
+  context.SystemJS = context.System = configLoader;
   context.global = context.GLOBAL = context.root = context;
   // make require available too, make it look as if config file was
   // loaded like a module