PHP 7.3 compatibility

See merge request !1

Per https://make.wordpress.org/core/2015/07/02/deprecating-php4-style-constructors-in-wordpress-4-3/, WP 4.3 deprecates PHP4-style widget constructors, and their usage will trigger deprecated notices. Switching to PHP5 constructors is simple and resolves the problem.

* Support for WordPress 4.2 and its splitting of shared terms
* Correct line endings in readme file

Fixes #1.

WP has set the expectation that arguments can be specified as strings joined by ampersands. While I think it's an unnecessarily-confusing way of specifying arguments, some users seem to like it.

See http://wordpress.org/support/topic/unable-to-customise-plugin-behaviour-in-non-widget-implementation.

Introduced in e696a372.

Introduce magic getter to correct fatal error introduced in 246be76c when visibility was added to class properties