Fixing line-height of list items in tree view.

The notebook dashboard has a list of notebooks.  In master, the notebook names have too much space below and not enough above.  This PR sets the `line-height` to center the notebook names vertically in the box.

Before:

![screen shot 2013-09-25 at 6 16 42 pm](https://f.cloud.github.com/assets/27600/1214773/7ac2132e-2649-11e3-8d6f-9614d4590089.png)

After:

![screen shot 2013-09-25 at 6 15 42 pm](https://f.cloud.github.com/assets/27600/1214774/7e5aa42e-2649-11e3-86b5-c9edd200c0fd.png)