Added highlighting for the currently selected tag

Fixes #11 

Opted for a light blue color, which I don't think is too intrusive. I've attached a screenshot of how it looks when the mouse is hovering over it, as well as it's normal color.

![dark-highlight](https://cloud.githubusercontent.com/assets/2644780/3436215/16cbaa9e-009e-11e4-8903-7d9739ed3f64.png)
![light-highlight](https://cloud.githubusercontent.com/assets/2644780/3436216/16cc453a-009e-11e4-83cb-3f981645e62b.png)

As discussed on HipChat, it would be great if this could be written in a more React-y, declarative way. Try to avoid using jQuery. The active state of a tag should be determined just based on the `this.state` or `this.props` of whatever component renders the tag itself, not this `Page` component.

Also, we use https://github.com/rpflorence/react-nested-router which has an auto-active-class thing. Not sure if that can be used for this though, but worth looking into.