Features
- added shortcode 'redirect_to_random_button' (with optional parameters 'div_class', 'button_class', 'text', 'redirect_to' and 'params' - documentation)
- added parameters 'utm_source', 'utm_campaign', 'utm_medium', 'pk_campaign' and 'pk_kwd' to support tracking redirects
- more robust against interference from themes through filter hook (effective only if redirect_to is called)
Other
- switched to the more flexible 'WP_Query' class
- confirmed to work with WP 4.5
