Converts the document to a string.

def to_string(self):
        """ Converts the document to a string. """
        buff = u""
        for child in self.content.iter():
            if child.tag in [self.qn('text:p'), self.qn('text:h')]:
                buff += self.text_to_string(child) + "\n"
        # remove last newline char
        if buff:
            buff = buff[:-1]
        return buff