added Boolean Codec, other special attributes
change arguments of Boolean codec
raise when Boolean.encoding or decoding text was invalid
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