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/cloud/cloudCodeAbsoluteFile.js

@@ -1,3 +1,3 @@
-Parse.Cloud.define('cloudCodeInFile', (req, res) => {
-  res.success('It is possible to define cloud code in a file.');
+Parse.Cloud.define('cloudCodeInFile', () => {
+  return 'It is possible to define cloud code in a file.';
 });