Update HiDPI CSS `@media` rule

Provide greater cross-browser support. Thanks to Wordpress -
http://core.trac.wordpress.org/changeset/22629 - and @marcedwards - see
issue #1127.
Adjust CSS image-replacement

Make sure that any text-highlight, or other factors that may break the
illusion, are far enough outside the element to be unnoticed.

Fix gh-1239
Remove named function expression in plugins.js

Details on various bugs with named function expressions:
http://kangax.github.com/nfe/#jscript-bugs
Minor amend, closing a missing bracket.
HiDPI (32x32px) favicon.ico

32x32-only version. Tested in IE6+, Safari 4+, and the latest Chrome,
Firefox, and Opera.
Merge pull request #1286 from jonathantneal/issue-1285

HiDPI (32x32px) favicon.ico - Issue #1285
Update CHANGELOG
v4.0.3
Mention Normalize.css in humans.txt

Normalize.css was missing from the stock humans.txt's technology
colophon.
Improve copy in `CONTRIBUTING.md`

Reflect some of the recent changes in necolas/issue-guidelines that tidy
up the language and simplify parts of the guidelines.
Merge remote-tracking branch 'upstream/master'