Improve older browsers support

Basically

1. get rid of `background-size: contain`
2. use Bootstrap's `row-fluid`
3. switch to using `img` for the logo

Before:
![before](https://f.cloud.github.com/assets/349621/1765363/ff1e18c6-6734-11e3-82ac-d8a4f68033a0.png)

After:
![after](https://f.cloud.github.com/assets/349621/1765373/0b6cd4e6-6735-11e3-84df-e89351682325.png)