Retrieve all the authors used in posts
        and convert it to new or existing author and
        return the conversion.

def import_authors(self, tree):
        """
        Retrieve all the authors used in posts
        and convert it to new or existing author and
        return the conversion.
        """
        self.write_out(self.style.STEP('- Importing authors\n'))

        post_authors = set()
        for item in tree.findall('channel/item'):
            post_type = item.find('{%s}post_type' % WP_NS).text
            if post_type == 'post':
                post_authors.add(item.find(
                    '{http://purl.org/dc/elements/1.1/}creator').text)

        self.write_out('> %i authors found.\n' % len(post_authors))

        authors = {}
        for post_author in post_authors:
            if self.default_author:
                authors[post_author] = self.default_author
            else:
                authors[post_author] = self.migrate_author(
                    post_author.replace(' ', '-'))
        return authors