Convert `news` to `_posts` format

Features:

* Post listing at `/news`
* RSS through atom (`/atom.xml`)
* Individual posts (<date>/<post slug>). RSS points at these.

I changed base layout resources to point at the site root. This was done to keep `post` layout simple (derived from base).

It is possible to generate new posts using `rake post title="First Post"`. Obviously you should have `rake` installed for this to work.