Update Test page to use ParseJS SDK

Currently, the [test](https://github.com/parse-community/parse-server-example/blob/master/public/assets/js/script.js) page uses native JS to connect to Parse Server.

One of the strengths of Parse is the great SDKs. I propose updating this file to use the JS SDK and give users an example of how to connect, save, find, etc.