Add pinterest button and reorganize social sharing

First step in better social experience
I created one template for each social media (facebook, twitter, g+ and pinterest)
Also social media are only displayed when information is available

Finally, I corrected the facebook template which was using some unknown app_id...... that is user specific and needs to be set per user!
Should the `li` be optional here too, as if it's not enabled via config there will be blank `li`s?