Combine the `html` selectors into one rule

Close #1492.
Add `Disallow:` to `robots.txt`

The addition of `Disallow:` is made in order to be compliant with:

  * the `robots.txt` specification (http://www.robotstxt.org/), which
    specifies that: "At least one Disallow field needs to be present
    in a record"
  * what is suggested in the documentation of most of the major search
    engines, e.g.:

      - Baidu:  http://www.baidu.com/search/robots_english.html
      - Google: https://developers.google.com/webmasters/control-crawl-index/docs/getting_started
                http://www.youtube.com/watch?v=P7GY1fE5JQQ
      - Yandex: help.yandex.com/webmaster/controlling-robot/robots-txt.xml

Besides the addition specified above, this commit also:

  * adds a comment making it clear to everyone that the directives from
    the `robots.txt` file allow all content on the site to be crawled
  * updates the URL to `www.robotstxt.org`, as `robotstxt.org` doesn't
    quite work:

      curl -LsS robotstxt.org
      curl: (7) Failed connect to robotstxt.org:80; Operation timed out

Close #1487.
Further optimize `apple-touch-icon-precomposed`

  before:         1226 B
  after:          1076 B
  ──────────────────────
  reduction:      -150 B [-12.23%]
Update to Normalize.css 3.0.0
Update `CHANGELOG.md`
Update to jQuery 1.11.0

Close #1499.
Docs: Add `Separate mobile URLs` subsection

Inform users that, if they use separate URLs for desktop and mobile
users, they should consider helping search engine algorithms better
understand the configuration on their web site.

See also:

 * https://developers.google.com/webmasters/smartphone-sites/details#separateurls
 * https://developers.google.com/webmasters/smartphone-sites/feature-phones

Close #1504.
Make consistency changes

Change code indentation in `normalize.css` from 2 to 4 spaces. This
is done in order to be consistent with the indentation used in the
other files.
Update to Apache Server Configs 2.2.0