Rebranding proposal

This is a first proposal for the site rebranding based on Twitter's Bootstrap.

**This PR is just a WIP. DO NOT MERGE!**

Note the branch only exists in GitHub (not in the ASF repos). This way we can add changes to the branch and use the pull request to discuss the new layout, design, theme, etc, and have it online so we can see how it evolves. Consider this branch yours, and feel free to commit directly any change you want. Let's build together the new site!

I've made the first steps to rebrand the site using the [Flatly](http://bootswatch.com/flatly/) theme:

* Configured a more friendly landing page. It's still missing an initial set of icons of the popular supported providers.
* Configured the main navigation bar with the pages I think are the most important ones. Any main guide should have a direct link there.
* Added the links to the "Getting started" guides. The pages need to be reworked, though.
* Migrated the Blog.

I've changed a bit the structure of the site. I've moved the `gettingstarted`, `blog` and `documentation` folders to the top level, to have a directory structure that matches the navigation bar. That would help contributor find the documents easily. All documents from the old site have been moved to the `old-site` folder, so we can progressively add and re-style them.

And that's it! We should start by discussing about the new layout (navigation, the structure of the landing page, and the pages), and once we are happy with the structure we can move to the theme/style, as it should be as easy as replacing a CSS file with the one we prefer.

@everett-toews @demobox Wanna have a look and spend some time with this?