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)

Sorry to be confusing, but the `Grays` section in `_variables.scss` is part of Bootstrap and not the [Solarized](http://ethanschoonover.com/solarized) color scheme which we use. The `Solarized color scheme` section marks the start of that. Sorry about the confusing organization and dead bootstrap variables which should be removed.

So please try one of the `$content-*` colors or `$bg-*` colors. Hopefully that should work, but if not, you can try Sass `mix` to get the gradient you want.

Can you try one of the Solarized color scheme colors? The `Grays` section isn't part of that