Added migrator to import entries from Serendipity (S9Y)

* Only uses classes from Ruby's standard library, no database required
* Entries can be exported from http://blog.example.com/rss.php?version=2.0&all=1
* Usage: ruby -r './s9y_rss.rb' -e 'Jekyll::S9Y.process("http://blog.example.com/rss.php?version=2.0&all=1")'