Adds support of displaying sharing count for multiple instances

per @kraftbj :
> By default, sharing_display() should only appear once per post, though users may want to insert multiple instances (before and after content, for example).

> Since the sharing counts are inserted based on the element's ID, jQuery will only insert the count to the first instance of the sharing buttons.

> Instead of using ID, we could use a class instead and insert it into all instances of a particular post's sharing buttons.

In order to support multiple instances I had to add a class (same as id attribute) and tweak the JavaScript the way it supports not id, but class.

JavaScript now uses $.each() method as simple class selector alone is not working as it is supposed to append the sharing count to first child span element.

work towards: https://github.com/Automattic/jetpack/issues/1234