Add retina support for icons & logo

I converted all the icons to [font-awesome](http://fontawesome.io) vector icons for better display on retina devices.

two things to note:

1. I did a pretty quick and dirty job of up-converting the logo, even though it might look OK. I figured you might want to replace mine with a larger copy of the original PSD.
2. script.js had a bunch of of jQuery UI `.button()` calls to convert the anchors to buttons. I just got rid of the `icons` parameter on them all, but I could clean this up in a few different ways:
  1. get rid of the jQuery UI calls... just give buttons a special class and style them in CSS (my preference)
  2. give buttons a "button" class and make a single jQuery UI call on that selector
  3. go back to the old way, and make jQuery append the retina icons in whenever creating the buttons