Improve readability of nav links in the header nav

Might be just me, but I'm thinking this increases readability of top navigation links by quite a bit.

### Regular before
![Screen Shot 2013-04-10 at 19 28 29](https://f.cloud.github.com/assets/380/363464/6ff62ab2-a204-11e2-8e01-e324c3135351.png)

### Regular after
![Screen Shot 2013-04-10 at 19 28 53](https://f.cloud.github.com/assets/380/363465/700fa50a-a204-11e2-98a9-cb98469e9340.png)

### Hover before
![Screen Shot 2013-04-10 at 19 29 47](https://f.cloud.github.com/assets/380/363466/7029640e-a204-11e2-809d-31ab94d2c645.png)

### Hover after
![Screen Shot 2013-04-10 at 19 30 05](https://f.cloud.github.com/assets/380/363467/70354f80-a204-11e2-926a-892db425ea22.png)