Style updates for navbars

Small style tweaks to navbars to bring them closer to webmaker.org

For all navbars:
* Nav items have `18px` of horizontal padding rather than `15px`

For `.navbar-inverse`, we need

* `font-weight: normal`, not light
* Webmaker.org has `-webkit-font-smoothing:antialised` applied
* default state (no hover) is #FFF with 100% opacity

Current:
![image](https://f.cloud.github.com/assets/1455535/2400059/c8cadbb0-aa05-11e3-88fc-257ceb8aee5a.png)

Intended:
![image](https://f.cloud.github.com/assets/1455535/2400064/d7d2f322-aa05-11e3-8a49-eb8a63e1a2d0.png)