Don't initialize menus until DOM is loaded

[bamadesigner]

Description

This PR addresses issue #95 by ensuring menus aren't initialized until the DOM is loaded.

I also optimized a few lines of logic, to change to test if certain variables are "falsey" instead of strict comparing for undefined because "falsey" will cover undefined and null.

List of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• This pull request relates to a ticket.
• My code is tested.
• I want my code added to WP Rig.

[hellofromtonya]

Solid improvements 👍. Needs a fix for Travis and then ready to fly.

[bamadesigner]

@felixarntz Travis is failing and its some kind of PHP issue? Morten said you might be the one to ping to help me figure it out.

[felixarntz]

@bamadesigner The problem is already caused by the current master branch. #146 aims to fix it. After that is merged and you merge it into your branch, it should work properly.