Reorganization of the guides in subcategories

I have reorganized the guides into more targeted categories:

- Integrate Piwik (HTTP and JS API)
- Develop Plugins
- Develop Piwik (contributing)

The idea is to make it easier for a visitor of the website (a developer) to find what he's looking for based on what he is trying to do.

For example the guy who develop his plugin doesn't necessarily want to learn about the HTTP API or Javascript tracker or Piwik's Core team workflow (but if he does he'll know where to find the docs of course).

Screenshot of the current layout and organization:

![old](https://cloud.githubusercontent.com/assets/720328/4452079/0505c214-4841-11e4-9246-ef00b9802630.png)

Screenshot of the proposed layout and organization:

![new](https://cloud.githubusercontent.com/assets/720328/4452080/0e35f3cc-4841-11e4-8ed4-38b87878f62e.png)

Urls are preserved: guides are still accessible at http://developer.piwik.org/guides/getting-started-part-1 however http://developer.piwik.org/guides is replaced by 3 categories (the one listed at the top). http://developer.piwik.org/guides redirects to the home.

I have also taken the liberty to throw in a Bootstrap theme, but it can be changed to another one if you don't like it.