Returns all keys that have been changed.

        If the keys are in child dictionaries they will be represented with
        . notation

def changed(self):
        '''
        Returns all keys that have been changed.

        If the keys are in child dictionaries they will be represented with
        . notation
        '''
        def _changed(diffs, prefix):
            keys = []
            for key in diffs.keys():
                if not isinstance(diffs[key], dict):
                    continue

                if isinstance(diffs[key], dict) and 'old' not in diffs[key]:
                    keys.extend(_changed(diffs[key],
                                       prefix='{0}{1}.'.format(prefix, key)))
                    continue
                if self.ignore_unset_values:
                    if 'old' in diffs[key] and 'new' in diffs[key] and \
                            diffs[key]['old'] != self.NONE_VALUE and \
                            diffs[key]['new'] != self.NONE_VALUE:
                        if isinstance(diffs[key]['new'], dict):
                            keys.extend(
                                _changed(diffs[key]['new'],
                                         prefix='{0}{1}.'.format(prefix, key)))
                        else:
                            keys.append('{0}{1}'.format(prefix, key))
                    elif isinstance(diffs[key], dict):
                        keys.extend(
                            _changed(diffs[key],
                                     prefix='{0}{1}.'.format(prefix, key)))
                else:
                    if 'old' in diffs[key] and 'new' in diffs[key]:
                        if isinstance(diffs[key]['new'], dict):
                            keys.extend(
                                _changed(diffs[key]['new'],
                                         prefix='{0}{1}.'.format(prefix, key)))
                        else:
                            keys.append('{0}{1}'.format(prefix, key))
                    elif isinstance(diffs[key], dict):
                        keys.extend(
                            _changed(diffs[key],
                                     prefix='{0}{1}.'.format(prefix, key)))

            return keys

        return sorted(_changed(self._diffs, prefix=''))