Updated backend to build iPython notebooks into static HTML

I've added a `setup.py` script to run & build the iPython notebook tutorials into static HTML pages for the tutorial site. New features with this PR:

- Each tutorial directory now must have a `metadata.yml` file. This is a [YAML](http://www.yaml.org/) file with some extra information about the tutorial, e.g. the author's name, approximate date it was written, name of the tutorial to appear in the `index.html` file, and a boolean flag that specifies whether the tutorial is published (see [here](https://github.com/adrn/astropy-tutorials/blob/master/tutorials/FITS-Header/metadata.yml) for an example)

- The deploy script (`deploy.sh`) is no longer run by travis and now must be manually run (by me / the repo admin) to push updates to the `gh-pages` branch. 

Future PR's will:
- Add documentation for how to contribute a tutorial,
- Add a description of the tutorial system for the `index.html` page,
- Add custom CSS to make the notebook style match the main astropy site

cc @astrofrog @eteq @kelle @migueldvb @jfoster17