'Add a field to store custom tags.
:param name: the name of the property the field is accessed
through. It must not already exist on this class.
:param descriptor: an instance of :class:`MediaField`.'

@classmethod
def add_field(cls, name, descriptor):

    if (not isinstance(descriptor, MediaField)):
        raise ValueError(u'{0}    must    be    an    instance    of    MediaField'.format(descriptor))
    if (name in cls.__dict__):
        raise ValueError(u'property    "{0}"    already    exists    on    MediaField'.format(name))
    setattr(cls, name, descriptor)
