Reduce the size of the 404 page

Changes:

* simplify the overall design
* remove Google search functionality as it does not provide much value in most
  cases
* remove unnecessary tags in order to reduce the file size (the 404 page is
  intended more as a placeholder and developers should change or customize it
  according to their needs)

Benefits include:

* small file size (994 bytes / 524 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 forgot 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+