Docs: update FAQ

Remove the section on "profiling" which was incomplete and not directly
linked to the code in this project. Also fix the headings so that
they're all of the same level.

Fix gh-1170
Add a HiDPI example media query

`-webkit-device-pixel-ratio` targets WebKit browsers.
`min-resolution` targets everything else.

Reference: http://www.w3.org/blog/CSS/2012/06/14/unprefix-webkit-device-pixel-ratio/

Fix gh-1127
Update to Normalize.css 1.0.1
Minor update to clearfix
Minor update to README
New image replacement approach

This approach uses a pseudo-element to force the element's content
downwards without covering the background of the element. The overflow
is hidden. For IE 6/7, fallback to the cruder `text-indent` method.

Known advantages:

* Works in IE6+ (although better in IE8+).

* Replaces any content in IE8+, including inner HTML.

* Nothing new for people to learn. Works just like all "traditional" IR
  techniques (unlike NIR, which needs you to add the image using a
  pseudo-element's `content` and relies margins for sprite positioning).

* Doesn't draw a large out-of-element box in modern browsers. You can
  even mix in something like `font: 10px/1 a` to reduce ce the size of
  the "off-screen" box, if you really need to.

* Doesn't have any potential SEO problems from `font-size:0`.

* Doesn't care about any minimum font-size that a browser might have.

* Doesn't have any potential failed-IR problems from inherited styles,
  like text being positioned within the element's visible box (i.e., if
  you use other properties like `text-stroke`).

Known issues:

* Doesn't work when images are off or fail to load (same as every other
  IR technique apart from NIR).

* If the IR'ed element has bottom-padding, then either it needs to be
  removed or the height of the pseudo-element needs to be bumped up
  (e.g., set to 200%).

* Doesn't avoid the `inline-block` bug in IE 6/7 due to the text-indent
  fallback for those browsers.

* Doesn't work on input elements (but they shouldn't be the subject of
  IR anyway).

* There is the potential for some final-result differences between
  IE 6/7 and modern browsers, but this is already the case with other IR
  techniques.

Fix gh-1149
Correct typo in the clearfix code
More general compression configuration for Apache

Works with Apache ≥ 2.1 and requires `mod_deflate` and `mod_filter` to
be enabled. However, use of Apache < 2.2 is no longer recommended.

For Apache ≥ 2.1 → 2.3.7, `mod_filter` isn't needed, but requiring it
and using this solution is the best way going forward (plus, it doesn't
introduce other dependencies to other modules like, for example,
`mod_version`).

Fix gh-1012
Ref gh-1173
Formatting headings.
Adding link to issues from Changelog

The Regex is 
Find: (#([0-9]+)) 
Replace: [$1](https://github.com/h5bp/html5-boilerplate/$2)
Adding a link to Issue Tracker in Reporting Issues section
Remove duplicate line from htaccess

Fix gh-1177
Remove placeholder link styles

The placeholder link styles don't add much value but do introduce
potentially unwanted specificity issues. By specifying `a:visited` it
forces subsequent styles for components (like buttons) to have a greater
specificity than this selector if they are to avoid the color changing
once it has been visited.

A similar problem is also evident if a user builds their link styles
upon the project styles (rather than editing the ones included).

This was also an issue in Normalize.css and was fixed in the same way.
Small adjustments to documentation
Fix broken links to issues in CHANGELOG
v4.0.0
Update to jQuery 1.8.1
Fix typo in .htaccess documentation, fixes gh-1198
Update Modernizr to v2.6.2

See http://modernizr.com/news/modernizr-262/ for more information on what changed since v2.6.1.