Add class to widget's UL for easier CSS

To allow this widget to be styled differently than other widgets it needs a class attribute to identify it with CSS. The idea came from WordPress's own [recent-comments widget](https://github.com/WordPress/WordPress/blob/master/wp-includes/default-widgets.php#L692), which has a UL element with an id attribute: recentcomments. This is obviously most useful if there are styles you want to apply to only this widget and not the others.

**This commit adds the "wp-most-popular" class to the UL element.**

No class is added to the LI elements because they can be selected as children of the UL element in the style sheet. This is the same as my previous pull-request except the class is wp-most-popular instead of most-popular.