Add atom.xml template for generating RSS feed
Reformat footer.html with all spaces

It was mixed spaces/tabs.
Enable configurable Google Analytics tracking

By setting `ga_tracking_id` and `ga_domain` in _config.yml, the tracking
code in the footer is generated with the proper configuration. If either
setting is omitted, the Google Analytics block is not rendered.
Merge pull request #5 from ssorallen/master

RSS feed and Configurable Google Analytics