Support application/rdf+xml as feed URL

By this change, the following MIME types will be supported:

* application/atom+xml
* application/rdf+xml
* application/rss+xml

I just update FeedSearcher to v0.0.4, which supports what this title says.
For more detail on MIME types, my recent blog post might help you.

* http://r7kamura.hatenablog.com/entry/2013/03/18/012636
* https://github.com/fastladder/feed_searcher/compare/v0.0.3...v0.0.4