Fixes issue #20

MarkupField now has 2 new (pseudo) protected properties:
`_markdownfield_rendered` and `_markdownfield_choices`. They contain
the CharField/TextField used to represent the instances for this
Markup Field.
`MarkupField.contribute_to_class` now checks whether one of them is set.
If not (and only if not), contribute_to_class will set these properties
and add the fields to the model.