Notebook server extension for activating/deactivating notebook extensions

This PR adds a notebook server extension that listens to `/nbextensions/` and displays a page listing all notebook extensions. Notebooks can be activated/deactivated on this page.

It consists of three parts:

### 1. YAML config files
Each notebook extension needs a YAML file to describe it. Here is an example:
```Python
# IPython Notebook Extension Description
Name: Runtools
Description: Runtools provide a number of afunctions for working with code cells
Link: https://github.com/ipython-contrib/IPython-notebook-extensions/wiki/Runtools
Icon: icon.png
Main: main.js
```
### 2. The server extension nbextensions.py
When requesting `/nbextensions/` all YAML files in the nbextension subdirecory are scanned and a list of extensions is sent to the page. The page template is called `nbextensions.html`

### 3. A Javascript handler
Handles the buttons on the pages and activates/deactivates notebook extensions

It's not very pretty right now but a start.