Add static, full-width navbar variation

Created by: Dentxinho

Navbars with class navbar-fixed-top or navbar-fixed-bottom have border-radius 0, while regular navbars have some border-radius... IMO if it is a navbar, it must have non-rounded corners (see Github's above!).

Fixed navbar: http://jsfiddle.net/aM2hq/6/

Non-fixed navbar: http://jsfiddle.net/aM2hq/5/