Stylesheet was not being served on an https enforced site because the URL was coded as an http URL. Stylesheets should also be enqueued on the wp_enqueue_scripts hook.

* Removed setting of WP_CONTENT_URL and WP_PLUGIN_URL constants. If these are not set, chances are the site will not even get to the point where your plugin executes.
* Moved stylesheet from the wp_head action to the wp_enqueue_script action
* Enqueued the stylesheet instead of just calling it inline.
* Calling plugins_url() is also much better than calling the constant DD_PLUGIN_URL.
* Changed the DD_PLUGIN_URL constant to be set by a plugins_url() call to ensure it receives https when appropriate.
Merge branch 'hotfix/5.3.1'
Merge pull request #12 from desrosj/master

HTTPS fix.