Navbar refactor

* Now responsive by default (fits better with mobile-first approach)
* Requires `.nav-header` to group `.navbar-brand` and `.navbar-toggle`
for proper mobile display.
* Changed `.nav-collapse` to `.navbar-collapse`
* Simplified examples in docs
Merge branch '3.0.0-wip' into bs3_navbar_overhaul

Conflicts:
	components.html
	dist/css/bootstrap.min.css
invert dropdown styles in navbar for inverse variation
lighter default navbar, add borders to navbars, tweak padding and alignments
Merge branch '3.0.0-wip' into bs3_navbar_overhaul

Conflicts:
	dist/css/bootstrap.min.css
Merge pull request #9403 from twbs/bs3_navbar_overhaul

BS3: Navbar overhaul