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

less/dropdowns.less

@@ -21,6 +21,11 @@
   position: relative;
 }
 
+// Prevent the focus on the dropdown toggle when closing dropdowns
+.dropdown-toggle:focus {
+  outline: 0;
+}
+
 // The dropdown menu (ul)
 .dropdown-menu {
   position: absolute;