Add directory navigation to dashboard

OK, this is what happens when the Super Bowl turns out like this...

I talked to @fperez today and we both feel that it is important to have some sort of minimal directory navigation in the dashboard for 2.0. This is an attempt to do that with a minimal amount of technical debt. Here is the rough outline:

* I have added directory items to the `FileNotebookManager`s list of notebooks. This is localized into two methods that can be easily removed once we have the full contents web service.
* The order in the dashboard is this:
  - `..`
  - 'index.ipynb` if it exists (this is case independent)
  - Then all directories sorted.
  - Then all notebook sorted.
* I have added icons for notebooks and directories and added the `.ipynb` to notebook items, like we do for nbviewer.

Here is a screenshot:

![screen shot 2014-02-02 at 6 21 42 pm](https://f.cloud.github.com/assets/27600/2062073/8cd65f1a-8c7a-11e3-9a1e-9ea98b13ac3e.png)