Override the Bootstrap responsive nav

I tend to use dev tools docked to the right of my window, so I removed the responsive styles that were kicking in for the fixed nav at narrower widths.

![marionette inspector pr](https://cloud.githubusercontent.com/assets/1902312/5145581/a5d35caa-7169-11e4-8ff0-de48912ee853.png)

For now, I cleaned out some unused classes and just threw in some CSS into `main.scss` to override the Bootstrap defaults. I don't know if this should live in a different file, but it seemed good for a quick fix. It might be fair to even throw out the Bootstrap stuff for some of these elements, if they aren't really going to use that much of the Bootstrap code.