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.
Fix typo in doc/css.md
Remove phrase about IE conditional classes
Update to Apache Server Configs v2.0.0
Update to Modernizr 2.7.0
Remove image replacement helper class `.ir`

This class was removed because it doesn't provide enough use
cases within modern web development. Cases are mostly handled
indiviually by developers.

Reference #1472.
Closes #1475.
Docs: Update server configs links

* Replace links to the Apache Server Configs `doc`
  directory, as it no longer exists:
  https://github.com/h5bp/server-configs-apache/commit/4f778c4a22da91721bc37c2eb10989b902ac4429.

* Provide missing links under the `X-UA-Compatible`
  section in the `html.md` file.
Docs: Update links and reorganize `extend.md`

* Update various links.
* Remove link to the `Node build script` repository as the project is no
  longer maintained.
* Better organize the information provided in `extend.md` by rearranging
  the sections alphabetically by title, and including a table of content
  for faster navigation.
Update Modernizr to v2.7.1