packaging tweaks
added _util.strftime to replace datetime.strftime
DataTypes no longer require a non-None default value for output
modfied tutorial.md regarding default values
added IStreamZlib adaptor
added SerialWriter.dump() method
added TabularReader.fields() method
added TabularWriter.fields() method
renamed some private classes
tweaked tutorial.md
trailing whitespace other than \n is preserved on input
v0.1.1