Add Lightbox

* Added Lightbox 2 (http://lokeshdhakar.com/projects/lightbox2/) with
  some custom modifications (sped up transitions, removed rounded corners)
Use lightbox on existing posts

* Added `rel="lightbox"` to all image links on existing posts (use
  recommended `data-lightbox` for future posts)
* Updated stylesheet to show zoom cursor for post images
Exclude _site folder from Sublime Text's search box

* Created a Sublime Project to exclude files/folders