add Boolean codec to codecs.py
add some special attributes to feed.py
change arguments of Boolean codec
change SaveOPMLError
raise when Boolean.encoding or decoding
text_type -> string_type. add test codes
code review
fix circular referrence problem.
make FeedList.__contains__() to use as "feed in feedlist"
change name OPMLXxx to OpmlXxx
remove unused attributes from FeedCategory
start working on docstrings
start documentation codecs.py
fix docstring
add docstring to Feed
mv feed.py to feedlist.py
add __all__ to feedlist.py