Fixed Gwikons & docs links, added a mixin, modified wrong selectors,

* Fixed link to docs found in groundwork.info. The correct link is
  http://groundwork.noceda.me/docs
* Modified ul in navigation.css.less to ul.menu
* Modified empty selector #navigation-wrapper. It should be #main-
nav-wrapper
* Added new css3 mixin .border-shadow(). It creates borders by using box-
shadow.
* Fixed mixins for Gwikons.
See http://groundwork.noceda.me/docs/changelog for complete log of
changes