Improve small screen experience

Not perfect, but a couple of quick improvements.

- Add device-width viewport
- Add max-width on logo
- Increase menu links click/touch area

References: #22 (Improve on iPhone)

![1](https://f.cloud.github.com/assets/1955/1332192/c74ee46c-3574-11e3-94dc-5e49d74f648a.png)
![2](https://f.cloud.github.com/assets/1955/1332187/c724bcc8-3574-11e3-8449-02300321aa67.png)
![3](https://f.cloud.github.com/assets/1955/1332189/c74a5442-3574-11e3-8f51-ddc7b592fb6b.png)
![5](https://f.cloud.github.com/assets/1955/1332190/c74d0d68-3574-11e3-8905-9372ded32a66.png)
![4](https://f.cloud.github.com/assets/1955/1332191/c74ee4c6-3574-11e3-8543-f066d085dc2e.png)