Bug fix suggestion about pypub

[dazuraz]

One more bug in the following method:

def _get_image_urls(self):
    image_nodes = self._content_tree.find_all('img')
    raw_image_urls = [node['src'] for node in image_nodes if node.has_attr('src')]
    full_image_urls = [urlparse.urljoin(self.url, image_url) for image_url in raw_image_urls]
    image_nodes_filtered = [node for node in image_nodes if node.has_attr('src')]
    return zip(image_nodes_filtered, full_image_urls)

Method urlparse.urljoin does not handle the case where self.url is a local file. Parameter full_image_urls will have the wrong values for any local file images.