Fix iTunes Parser

There are a few issues with the iTunes feed parser.

* ~~It appears it was deprecated and later reimplemented, but this change [isn't reflected in the specs](https://github.com/pauldix/feedzirra/blob/master/spec/feedzirra/feed_spec.rb#L71-81).~~ 7e23834 and 72a1a01b
* Sub-categories have [never been properly implemented](https://github.com/pauldix/feedzirra/blob/master/lib/feedzirra/parser/itunes_rss.rb#L32-35). See: #78