'created' is datetime.datetime object.
add Version codec
Merge branch 'codecs' into feedlist
add attribute 'version' to opml document.
initializing when FeedList created
Custom pytest assertion comparison for RevisionSet

http://pytest.org/latest/assert.html#defining-your-own-assertion-comparison
Make RevisionSet possible to be copied
Merge function for RevisionSet objects
Merge branch 'feedlist'
add example code to docstring
Merge branch 'feedlist'