Refine Mobile Styles

* Header has fixed position
* Update search bar placeholder text and styles
* Move "Repo" link into account list
* Decrease font size on mobile

https://trello.com/c/8lxiMPx3

---------

![hound-mobile-scroll](https://cloud.githubusercontent.com/assets/1174461/3884502/3e0c436a-21b0-11e4-9ce0-9426d28d969f.gif)