fix search interactions

More intuitive code search:
![2014-09-20 18_31_02](https://cloud.githubusercontent.com/assets/108094/4346787/d7e8bd04-4115-11e4-87a9-e1ca8c011fb2.gif)

Fix #735

- Search input persists if user dismisses and then re-triggers search
- If no match, 'no results' notice appears
- If no match, focus stays on search input
- Clicking 'no results' resets search input and clears notice. Otherwise, a new successful search will clear the notice.