set attributes for class.
	@param cls [any class]: the class to update the given attributes in.
	@param kw [dictionary]: dictionary of attributes names and values.

	if the given class hasn't one (or more) of these attributes, add the attribute with its value to the class.

def def_attrs(cls,**kw):
	'''
	set attributes for class.
	@param cls [any class]: the class to update the given attributes in.
	@param kw [dictionary]: dictionary of attributes names and values.

	if the given class hasn't one (or more) of these attributes, add the attribute with its value to the class.
	'''
	for k,v in kw.iteritems():
		if not hasattr(cls,k):
			setattr(cls,k,v)