Add atom.xml template for generating RSS feed
Drop unneeded tag selectors from .posts and #single-wrap

The class .posts and the ID #single-wrap are sufficiently specific to
match only the desired elements; the added specificity of a tag selector
is unneeded. This chops a bunch of bytes from the stylesheet.
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.