update and re-arrange contribute page

Ok, I've fully re-factored the contribute page. The features for 7 page was my inspiration. It has a few layout issues, I hope somebody can fix them - I couldn't. Otherwise, there is still room for improvement but it is far better than the current one so I'm for merging already!

@karlitschek @tomneedham @jancborchardt 
You could link to http://owncloud.org/contribute/design/ here :)