improve search form css and html

### Old  
![old](https://cloud.githubusercontent.com/assets/1080613/4415555/bd94d55c-452d-11e4-90e2-df6022a157e3.png)

### New  
![new](https://cloud.githubusercontent.com/assets/1080613/4415559/ce741b1c-452d-11e4-870e-e2d60a411a27.png)