UI For Metadata

This a prototype to edit metadata.
right now you need to execute 
<del>
```
%%javascript #execute to toggle header to edit metadata of cell
$('body').toggleClass('editmetaon')
```
</del>

To see the UI change.

I'll add a button to the toolbar to do so once #2127 is merged.
looks like
![Imgur](http://i.imgur.com/a215d.png)

Cell "header" are togglable by above command, and clicking on "raw edit" bring the modal editor.

My question is what kind of API do we want to add other buttons to the cell header/toolbar.

Obviously if an extension want to add a button, the button needs to be be added to all the current cell of a certain type. 

Do we allows extension to be loaded at any time, or do we says that some stuff can only be extended if the extension is loded at notebook load time? 

And we also need maybe an event to say that the metadata have changed.

## Update : 
see [this comment](https://github.com/ipython/ipython/pull/2333#issuecomment-11915191)
To see how to enable.