Add a .navbar-default class for default colors

* Extract color styles from .navbar { } and put them into
.navbar-default { }
* Change variable names
* Change the docs accordingly (i.e., add the .navbar-default class to
all default navbars)
* Change customizer accordingly
Fix some forgotten things for navbar-default

New variable names were missing here and there.
Add new navbar-default class to examples
Merge branch '3.0.0-navbar-default' of https://github.com/juthilo/bootstrap-wip into juthilo-3.0.0-navbar-default

Conflicts:
	dist/css/bootstrap.min.css
fix broken button