30277-Kienstra: Split customize-controls.js

This splits `customize-controls.js` into `customize-sections.js`, `customize-panels.js`, and `customize-utils.js`.

The customizer is now started with `wp.customize.init()`. It won't load unless you type that in your browser console. I'll work on passing the `_wpCustomizeSettings` object to it it in `customize.php`.

This moves the `api.Container` class to `customize-base.js`. This file has many of the base classes, and `api.Container` is a base class of `api.Panel` and `api.Section`. 

This might not be the right branch to request a pull on. This includes my code from #47 that has the `api.util` functions.

All of the JUnit tests still pass, though this is more of an integration issue.