docs(navigation): update style of navigation

**Old:**
![stlye-docs-navigation](https://cloud.githubusercontent.com/assets/746429/4175695/664db530-35e1-11e4-8260-73cf16085972.gif)

**New**:
![stlye-docs-navigation-new](https://cloud.githubusercontent.com/assets/746429/4175694/66422ba2-35e1-11e4-8f88-b1cf5f543dd9.gif)