[PR] Enhance handling of scripts and styles in the parent theme

* [x] Enqueues all scripts and styles on the proper hook for output with `wp_head()`
* [x] Calls for the WordPress version of jQuery, loads jQuery UI from Google
* [x] Introduces `spine_get_script_version()` to be used as a cache breaker
* [x] Removes assignment of `$ = jQuery`, as that should not be needed.