Add preparer for serialize

[digitalresistor]

Currently when you serialize values, there is no preparer step before passing it off to the sub-types (in the case of the MappingSchema)

colander/colander/__init__.py

Lines 2276 to 2293 in 141263a

	def serialize(self, appstruct=null):
	""" Serialize the :term:`appstruct` to a :term:`cstruct` based
	on the schema represented by this node and return the
	cstruct.
	If ``appstruct`` is :attr:`colander.null`, return the
	serialized value of this node's ``default`` attribute (by
	default, the serialization of :attr:`colander.null`).
	If an ``appstruct`` argument is not explicitly provided, it
	defaults to :attr:`colander.null`.
	"""
	if appstruct is null:
	appstruct = self.default
	if isinstance(appstruct, deferred): # unbound schema with deferreds
	appstruct = null
	cstruct = self.typ.serialize(self, appstruct)
	return cstruct

this means we can not influence the appstruct before passing it off to the various subtypes in the mapping schema

https://github.com/Pylons/colander/blob/master/colander/__init__.py#L811-L818

This doesn't match the deserialize API which has a preparer function that can be used to modify the cstruct before passing to children and or validation.