* Refactored code of finding VerseNode subclasses
* Subclasses of VerseNode needn't to have class attribite 'custom_type'
* WIP: Added __new__() method to VerseTag. It will be possible to
  define own VerseTag subclasses
* Fixed few bugs in verse_avatar.py and in test_node.py
WIP: subclasses of VerseTag