Add Grunt for Javascript compilation

Switch from .dev.js -> .min.js for minified files
Add composer.json

List phpunit/phpunit as development dependency.
Switch to current WP unit test suite

Courtesy of WP CLI scaffold
Define global plugin object using $GLOBALS

Fixes fatals during unit test runs.
Fix PHP warnings in test-library.php

Removes global / cache flushing functionality that is no longer needed after switch to current unit test suite.
Restructure test directories

Add note about selenium tests / private unit test suite fork.
Add testing docs to CONTRIBUTING
Fix PHPUnit command in docs
Update copyright
Adjust library tests to work around #29615

Non-page hierarchical post type 'Default Permalinks' do not work for child posts prior to 4.0.1.
Avoid stale cache in bu_navigation_load_sections
Fix pass-by-reference test error for PHP >= 5.4
Merge branch 'build' into develop
Send correct JSON headers with Ajax responses

Default response type of "text/html" was causing issues on TechWeb, where a navigation label of "<head>" was getting injected with script tags by our New Relic agent.
Limit allowed HTML tags in navigation labels

Uses `wp_kses_post` on save
Merge branch 'fix-INC11642932' into develop
Fix return value for getAncestors

And update navigation metabox to skip updating breadcrumbs if ancestors were not found. Previously getAncestors was returning the jstree container if the current section had not finished loading.