Responsive layout enhancements

Updated CSS with minor fixes to make site read better at narrower viewports.

![Before](https://cloud.githubusercontent.com/assets/41798/4020584/3aba5f34-2ac1-11e4-9552-9014280758ef.png)
![After](https://cloud.githubusercontent.com/assets/41798/4020585/4a73bef2-2ac1-11e4-96b8-cc284cae063b.png)