Search box fixes & improve desktop + tablet search form

- Fixes a regression caused by the inclusion of default form styles
- improve usability of the search form by capping its width
- Make sure header always goes on top of ads

## Before

![screen shot 2013-08-08 at 12 33 31](https://f.cloud.github.com/assets/85783/930985/66ed5238-001e-11e3-8025-aa1649babf66.PNG)
![screen shot 2013-08-08 at 12 33 04](https://f.cloud.github.com/assets/85783/930986/6999a540-001e-11e3-9f94-18d9780e0303.PNG)


## After

![screen shot 2013-08-08 at 12 31 03](https://f.cloud.github.com/assets/85783/930987/6eb5cbda-001e-11e3-821b-6b34cef5b7ec.PNG)


![Hum](http://i.imgur.com/nZsHdtG.gif)