Added RSS support and variables to control it.

Here's the new functionality:

* /feed.xml now contains an RSS feed that your readers can subscribe to.
* \_config.yml now needs a description field that provides a description of your site and a base_url field that specifies the protocol and domain it's served at--basically, the URL before the first /
* post descriptions are the post.excerpt field, which is auto-generated and [can be overridden manually](http://jekyllrb.com/docs/posts/#post_excerpts).
* your images are relative links, and will probably break. To avoid this, use {{ site.base_url }} before the path, and they'll become absolute URLs that don't break when you switch domains.
* the latest ten posts will be syndicated. If you want more or fewer, you can probably guess what you need to edit in feed.xml.

Sound simple enough?