Configurable GA, Atom RSS feed, and fewer CSS bytes

Thanks for the great theme. I wanted an RSS feed and my own Google Analytics settings and added them in this pull request.

* Move Google Analytics settings to _config.yml to let the user configure them. If either setting is missing, the `<script>` block is not inserted. That allows disabling GA if it's not wanted.
* Add `/atom.xml` and links to it in the head to provide an RSS feed of the blog.
* Simplify selectors that were using tag selectors when they didn't need to.