Clean up typeahead search styles.

![image](https://cloud.githubusercontent.com/assets/60455/5056395/0747eda8-6c50-11e4-95dc-dd15fa3d59a5.png)


  - More opaque background
  - Results are full width, have pointer cursor, and even padding
  - Don’t hide the search UI if you click inside the input

/cc @wsorenson