Add site news and associated atom feed

This PR adds a "news" page that displays a feed of posts (part of the discussion in ev3dev/ev3dev#109). There is also an atom feed of the same posts. I included a "lorem ipsum" news item to test with but we can (and probably should) replace it with an actual post before merging the PR.

To add this, there were a few larger things that I changed:
- I added a "community" drop-down that includes the news, project gallery, and contributing menu items. If I had added another without collapsing these, the nav would have been much too wide for smaller screens.
- I deleted the old `_posts` folder and moved the projects to a new `_posts` folder inside the existing projects folder. This both eliminates the need for explicit category definition (because Jekyll derives the set of categories by the directory structure above the posts) and allows us to add multiple post categories and keep them separate.

Neither of these changes should break anything technically, although we will need to update the "adding a project" guide with the new path.

My `gh-pages` branch has been updated with these changes as well.