add 'episodes' method

Implement the 'episodes' method that returns a list of entries in
the feed tha have an enclosure element.
workaround an annoying default in XML::Atom

XML::Atom returns a UTF-8 byte stream by default. We want Unicode
strings just like XML::RSS so we really can use XML::Feed as a
unified interface for both. Thus we set $XML::Atom::ForceUnicode.
add tests for feeds that use CDATA
add 'episode' method