visual fixes

1. Open external urls in new tab
2. Fix header layout on narrow screens
![image](https://cloud.githubusercontent.com/assets/812240/3655810/99f1e0be-117e-11e4-913f-efbc64522528.png)
3. Disallow line-breaks in captions
![image](https://cloud.githubusercontent.com/assets/812240/3655825/cb0cd28a-117e-11e4-94b5-6f474f870017.png)

Screenshots demonstrate current master. Please, review the changes