Adds Pinterest attribute to display share count. 

This will display the number of pins from images from the post displayed (e.g. if 5 images displayed on a page were shared a total of 10 times from Post A, it would display 10. If the same 5 images were displayed on another post (B), but never pinned from that page, it would display 0 on Post B).

Fixes Automattic/jetpack#597 cc: @enejb