Remove author as a required RSS field and use the description field instead, to reflect the RSS spec

Hi!

I noticed that the RSS generated by Cabin is not valid. This problem comes from the `rss` npm package.

According to the RSS spec there is no `<author>` attribute inside `<channel>`. Only `<item>` has this attribute, and it's optional. I've already reported [this](https://github.com/dylang/node-rss/issues/21).

The problem is that `grunt-pages` requires the `author` attribute to be present in the `options.rss` object. And the grunt task exits with fatal error if it's omitted.

So this pull request fixes that by replacing the `author` with `description`, as the latter is, according to RSS spec, a required field.

With this change, omitting the `author` attribute will generate a valid feed.