Migrate to new cloud code interfaces

removes job status object, moves messasge method on req object

Adds 3.0.0 migration guide

Fixes nits about 3.0.0 documentation

Adds update guide to README

spec/rest.spec.js

@@ -101,9 +101,8 @@ describe('rest create', () => {
   it('handles object and subdocument', done => {
     const obj = { subdoc: {foo: 'bar', wu: 'tan'} };
 
-    Parse.Cloud.beforeSave('MyClass', function(req, res) {
+    Parse.Cloud.beforeSave('MyClass', function() {
       // this beforeSave trigger should do nothing but can mess with the object
-      res.success();
     });
 
     rest.create(config, auth.nobody(config), 'MyClass', obj)