Add pelican-based webpage

Add a pelican-based website for DistArray.  It will allow us to easily add content in the future.  To see the generated website, see http://docs.enthought.com/distarray.

Requires the python package [pelican](http://blog.getpelican.com/).  Pelican lets us add content as restructuredText, Markdown, or AsciiDoc.  I also use the pelican theme [pelican-bootstrap3](https://github.com/DandyDev/pelican-bootstrap3).

I've split up and re-used `.rst` files from the docs as much as possible for the webpage.  I've integrated  @kwmsmith's `overview.rst` document from a branch of his.

As part of this PR, I add a `www` directory to `docs` and rearrange the `docs` directory a bit.  I put all of the sphinx documentation into a `sphinx` subdirectory of `docs`, and I add another subdirectory for `talks`.

If you have `pelican` installed, you should be able to go to the `docs/www` directory, type

```
make html
make serve
```

And go to `localhost:8000` to see the website locally.  `make github` will build and push the website to our `gh-pages` branch (requires `ghp-import`).  See the README document in `docs/www` for more.

Some content highlights
---------------------------------
1. Erick's DistArray logo
1. `features.ipynb` added as one of our pages (updated and improved in another PR)
1. Five DistArray talks added to the `Talks` page
1. Release notes added as a page