Fixed the button hover and active

The current implementation causes the layout below the button to jerk up and down when hovering or clicking it. This is because borders affect the element's offset height. My proposal is to use box-shadow to achieve the same effects without any side-effects.
Merge pull request #14 from GeKorm/patch-1

Fixed the button hover and active jerkiness for button:hover, #search input[type='submit']:hover, .wiki-page .wiki-page-content .wiki_button:hover