Add a basic Atom feed.

Based on the template at
https://github.com/middleman/middleman-blog/blob/master/lib/middleman-blog/template/source/feed.xml.builder
Add a link tag for the feed.
Merge pull request #127 from TimMoore/atom-feed

Add an Atom feed
Restore v1.5 index page.

Copied from
https://raw.githubusercontent.com/bundler/bundler-site/1b617fa496e8f5818eb923770bc2dcf2e947cfa0/source/index.haml

Fixes #124.
Merge pull request #128 from TimMoore/restore-v1.5-index

Restore v1.5 index page