Added favicon and SVG icons.

Hi!

Changes:
* I made a simple favicon for the site.
* Optimized the Google Analytics script in the `head` with the one from the html5boilerplate project.
* Added SVG icons. Most of them are from [thenounproject.com](http://thenounproject.com/) and look almost identical to the previous icons. I replaced the markdown links at the bottom of `archive.md` and `latest.md` with the svg ones.
* Ran the old icons through ImageOptim, which greatly reduced their filesize (in case you don't like the svg icons).
* Added an `attributions.html` file with the attributions the noun project asks for because I wasn't sure what else to do with it.

This is my first pull request, so hopefully I did everything right :smiley: