expose multiple exporters as url prefix, slides

Building on what was in #52 and #150, this adds the machinery for any number of additional exporters, exposed as top-level urls. The first additional exporter is `/slides/`, which uses SlideExporter. As a humorous side-effect,`/html/` will now also show the prior content.

![2014-11-02_1304](https://cloud.githubusercontent.com/assets/45380/4876218/c6ce699e-62ba-11e4-9836-23574683c0d3.png)

## UX
When viewing a notebook, a user can click on one of the other view buttons at the top of the screen, next to the notebook source. I'd like to add a slide link to the tree view as well, but pulling down every `.ipynb` to find out if it has metadata seems expensive. Also see "not reading metadata".

## What Works?
- basic rendering against all endpoints (gist, local, url, etc.)
- reveal.js themes
  - ![2014-11-02_1306](https://cloud.githubusercontent.com/assets/45380/4876280/18c7a0fa-62be-11e4-90da-6b7ed72bf5cb.png)

## What Doesn't?
- introspecting the metadata to figure out if there are actually are slides... it's not pretty if there aren't 
- `print-pdf` doesn't appear to work, at all. I have found that feature to be extremely frustrating in the past, and don't really care to debug the interaction between bootstrap, nbviewer, ipython and reveal as a blocker to getting this in, even though someone will ask for it immediately.

## Tornado
- Multiple exporters are configured at the `app.py` level.
- `handlers.py` doesn't know anything about them, in specific, but handles the URLs, redirects, etc. 
- `render.py` does need to know a bit, as, for example, the output of `SlideExporter` needs to have a lot of stuff stripped out (since all the `reveal_internals` were merged into `slides_reveal.tpl`), and it renders a whole page.

## Templates
- Some light refactoring of the notebook template minimizes the amount of duplication. Mainly adding more blocks.
- For slides, moved the breadcrumbs to the menu managed by Headroom so that it gets out of the way.
- Using FontAwesome throughout.

## Assets
I've followed the pattern of using the bower version of the component.
I've added this to the newest, rebased commit... still pretty broken. this is probably okay, as there's no UI to activate print-pdf.