add more info to rss feeds

[As requested](http://meta.discourse.org/t/rss-support-for-categories-topics/2075/68).

Additions to category RSS feed ([e.g.](http://meta.discourse.org/category/feature)):

* feed level (the category)
  * `<lang>` if it's found in site settings
  * `<lastBuildDate>`
* item level (the topics)
  * `<author>`
  * `<category`

Additions to topic RSS feed ([e.g.](http://meta.discourse.org/t/rss-support-for-categories-topics/2075/66.rss))

* feed level (the topic)
  * `<lang>` if it's found in site settings
  * `<lastBuildDate>`
  * `<category`
* item level (the posts)
  * `<author>`

I also tweaked the RSS content (`<description>`) a bit, in both feed types. Open to suggestions about what should and shouldn't be there.