HiDPI support

Made some improvements to the site's assets for better looking design on a HiDPI screen (e.g. Macbook Pro Retina). It would have been nice to include HiDPI versions of all imagery, but couldn't get my hands on some of the original graphics.

**Some of the changes include:**
- Icon font for resolution independent icons
- CSS3 buttons for resolution independent buttons
- @2x images for a crispier look on HiDPI displays