Fix classes

zippydoc/block_markup.py

@@ -1,7 +1,7 @@
 import re
 from value import Value
 
-class TreeLevel:
+class TreeLevel(object):
 	def __init__(self, indentation, data):
 		self.elements = []
 		self.indentation = indentation

zippydoc/document.py

@@ -1,7 +1,7 @@
 import re
 import block_markup
 
-class Document():
+class Document(object):
 	def __init__(self, data):
 		self.data = data
 		self._parse()

zippydoc/transformation_ruleset.py

@@ -1,4 +1,4 @@
-class TransformationRuleset():
+class TransformationRuleset(object):
 	def transform_children(self, text):
 		pass