Nbconvert HTTP service

This brings back print preview and 'download as' (currently Python, HTML and reST are in the menu).

New URL patterns:

* `GET /nbconvert/<format>/path/to/Notebook.ipynb`
* `POST /nbconvert/<format>` with the JSON notebook model as the body data.

nbconvert exporters gain a `mime_type` traitlet, used in the HTTP response headers so the browser knows what type of file it's getting.