Add an option to debounce the window resize event

The option is called `debounceResize`, and defaults to false.

If there are lots of text boxes where readmore is applied, resizing the browser window can be a performance killer.

This change adds a dependency to [Ben Alman's throttle/debounce plugin](http://benalman.com/projects/jquery-throttle-debounce-plugin/).

If the plugin is not present, the `debounceResize` option has no effect.