Added Share header w/ 'Tweet' and 'Like' buttons

Fairly simple commit. A couple of things I wasn't sure about:

* Which CSS file to use... as a result, I added the necessary css to both `application.css` and `layout.css`
* If the events show.html.erb is for some reason rendered without the item partial. Because I inserted the facebook like-button js into the `<% content_for :javascript_insert %>` and I don't want it to throw a js error if the dom elements are missing.

Let me know if you see anything you need fixed or changed!