Fix vertical spacing around lead ad slot

- [fix] Removes an unexpected amount of space around the lead ad on mobile
- [new] Slightly smaller top vertical space at higher breakpoints to match the bottom space

![screen shot 2014-03-24 at 15 06 09](https://f.cloud.github.com/assets/85783/2500773/f1d8c4f4-b365-11e3-83bc-a7971845b0d5.PNG)

![screen shot 2014-03-24 at 15 03 15](https://f.cloud.github.com/assets/85783/2500774/f4843792-b365-11e3-86f8-6f11d4d222ea.PNG)

![ ](http://media2.giphy.com/media/TW1oEbaZ6HRYY/200.gif)