Remove notes about Google Chrome Frame from docs

Currently we don't use Google Chrome Frame anymore as support for
it will end at the end of the year.
This commit removes references to Chrome Frame within the
documentation. It is related to 2cf1ef7.

Reference #1433.
Remove IE conditional classes

The reasons behind this decision include the following:

* This project will drop legacy browser support (see #1050), therefore,
  the use for conditional classes for IE 8+, becomes much more limited.
* IE 10+ does not support conditional comments:
  http://msdn.microsoft.com/en-us/library/ms537512%28v=VS.85%29.aspx.
* Users do and can develop easily without using the conditional
  classes, this technique being very limited in scope as no other
  browser versions are explicitly target in the same way.
* It fixes the issue that prevents IE from honouring
  `<meta http-equiv="X-UA-Compatible" content="IE=edge">` (see: #1187).

This change also removes the related documentation.

Close #1290 and #1187.
Update to normalize.css v2.1.3

Ref: #1050.
Remove IE 6/7 hacks from `main.css`

Ref: #1050.
Show `outdated browser` prompt to IE 6 and 7 users

Ref: #1050.
Update documentation and README.md

Remove content regarding legacy browsers.

Ref: #1050.
Fix comment in index.html about apple-touch-icons

Close #1438.
Docs: Add info about how to block Google Translate

Inform users on how to disable the translation prompt in Chrome or
block Google Translate from translating a web page or any particular
section of the web page.

See also: https://support.google.com/translate/?hl=en#2641276.

Close: #1440.
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 IE from displaying its own custom error page (thanks
  @mikealmond and @patrickkettner):
  http://www.404-error-page.com/404-error-page-too-short-problem-microsoft-ie.shtml

Benefits include:

* small file size (1409 bytes / 580 bytes gzipped), which is particular-
  ly 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+

Close #1443.
Add vertical centering for `svg`

Remove gap between the `svg` elements and the bottom of their containers
by vertically centering them.

Ref #1326.
Close #1453.
Docs: Remove outdated info on Skype highlighting

The snippet provided in the docs about how to prevent the Skype
plugin from formatting numbers on a web pages is outdated and no
longer works.

Nowadays, the Skype plugin makes it more difficult to overwrite
its injected styles (class names include timestamps, e.g.:
`skype_pnh_print_container_1381964168`, `!important` is used, etc.).

Providing a solution for this use case isn't viable. Also, users
that install such a plugin usually expect the behavior.

Close #1454.
Add note about legacy browser support in v4

Close #1050.
Fix typo in header of doc/html.md (promt → prompt)

Close #1459.