save wscorpus to files

        :param str folder: path to directory
        :type folder: string
        :param str format: either TEXT or COLUMN
        :type format: str

def save(self, folder, format):
        """save wscorpus to files

        :param str folder: path to directory
        :type folder: string
        :param str format: either TEXT or COLUMN
        :type format: str
        """
        try:
            mkdir(folder)
        except Exception:
            pass
        for document in self.documents:
            f = join(folder, document.id)
            content = u"\n".join(document.sentences)
            write(f, content)