Add navigation links in lessons (v5)

This is a **work in progress** for #214. **Don't merge it yet.**

## Abstract

Right now our lessons don't have a navigation bar with links to previous and next lesson as show in the screenshot below.

![swc-no-nav](https://f.cloud.github.com/assets/1506457/2429561/032c466e-ac8a-11e3-8a91-9e80f0db9d2b.jpg)

This PR add this useful navigation bar as show in the screenshot below.

![swc-nav](https://f.cloud.github.com/assets/1506457/2429567/2bd80f12-ac8a-11e3-987e-4d39dbd2e21f.jpg)

## Technical details

The navigation bar can be build using **only** Jekyll's template (but I fail to do it, you can check what I do in r-gaia-cs/bc@715c5bd) or adding the information needed in YAML header (what this PR does).

Add the information needed to the navigation bar in YAML header have the disadvantage that it will need to be update at some point (this isn't a big problem because `make` can do it) and for lessons wrote using IPython Notebook it need to be add after the `.ipynb` is converted to `.md`.

## TODO

- Improve the navigation layout using CSS

  I invite all CSS experts to send a PR to fix this.
- Improve the `Makefile` to handle IPython Notebook lessons properly

  @gvwilson Can you take a look on it?