Aesthetical tweak: round color samples instead of squarish ones

See http://i.imgur.com/czJUkTH.png for a preview.
Only tested with Chrome 39.0.2171.71 (64-bit)
Remove obsolete CSS rules
Better classes and var names

Also add CSS comments to explain how to modify the color sample's look.
More shapes ideas: http://css-tricks.com/examples/ShapesOfCSS/.
Merge pull request #7 from abourguignon/master

Customizable color samples
Add @abourguignon to contributors