Twitter card

This addresses #18 and #164.

The twitter card is very basic, setting defaults in required options.

For optional values, the template will need new values (defaulting to nothing otherwise).

# The full list of names for the jinja namespace

* description - Meta tags `description` and `twitter:description`
* author - Meta tag `author`.
* notebook_title - Meta tag `twitter:title`
* thumbnail - Image link that will appear in `twitter:image:src`, defaults to https://raw.github.com/ipython/ipython/master/docs/resources/ipynb_icon_256x256.png