[WIP] Use a RailsBridge-y navbar.

I'm not sure shared nav as discussed in #243 is necessary but as part of a refactor before I start on some new curriculum I wanted to get the header bar looking more like RailsBridge.org does now.

![curriculum header](https://f.cloud.github.com/assets/358882/1274241/254cff52-2d84-11e3-906a-6695efccdf78.png)

Here's a quick first go. I did some brutal hacks to make this happen and I'll need to look closely at the CSS and erector Ruby before this is anywhere near ready. Just want to solicit feedback on the direction and approach.

/cc @lilliealbert @rachelmyers @tjgrathwell