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 
Ah there is already a page-events.php. Just make a page in your wordpress admin called 'Events' (make sure the slug is 'events') and it will load the content from page-events.php