Remove margin between buttons in button groups; use border-radius mixins to avoid reseting border-radii that don't need resetting

Additional minor updates to the docs

Minor fixes to docs

* Change .brand to .navbar-brand
* Change .btn-navbar to .navbar-toggle
* Redesign navbar toggle to be a bit lighter and wider
* Center align the navbar brand with a max-width of 200px (to minimize hit area) while still centering

Bring back the label, make it scale somewhat with the font-size of the parent using percentages and ems