changed to use button_to_add instead of link_to_add to make it more sema...

By reading the article by Heydon http://coding.smashingmagazine.com/2013/08/20/semantic-css-with-intelligent-selectors/. And using the https://github.com/Heydon/REVENGE.CSS. 

We would see the warning message for link_to_remove is something like: "Do you mean for this to be a link or a <button>? It does not link to anything!"

And warning message for link_to_add is something like "If you are going to make it look like a button, make it a button, dammit!"

Therefore, I decided to make a PR for this. Using button instead of link to make it more semantic.