Redesign 404 page

Changes:

* simplify the overall design
* remove the Google search functionality as in most cases, it does not
  provide much value
* reduce the overall file size while keeping the gzipped size over 512
  bytes to prevent Internet Explorer from displaying its own custom
  "friendly HTTP error page":
  http://blogs.msdn.com/b/ieinternals/archive/2010/08/19/http-error-pages-in-internet-explorer.aspx.

Benefits include:

* small file size (1242 bytes/580 bytes gzipped), which is particularly
  useful when users are on a data plan and / or the server is wrongly
  configured (e.g.: tries to redirect to a "mobile" version of the web
  page and ends up serving a 404 page) or developers forget to provide
  certain files (e.g.: /favicon.ico -
  http://zoompf.com/blog/2012/04/instagram-and-optimizing-favicons)
* improved performance (less HTTP requests, less memory consumption,
  faster rendering, etc.)
* designed to work with all kinds of screen sizes and support IE 6+
Merge pull request #202 from h5bp/update-404

Redesign 404 page