fix various JSHint warnings
fix typo
jquery.fancybox.css: put separate elements in a new line
README.md improvements:

* use Markdown syntax where possible
* break long lines
* fix typos
Optimize images with kraken.io and with https://github.com/subzey/zopfli-png.
jquery.fancybox-buttons.css: Remove unit from 0.
css: use tabs for consistency.
Update jquery.mousewheel to v3.1.3.
CHANGELOG.md: use two blank lines before each version for consistency.