Add __all__ to w3lib.url package

Fixes #54

Author: redapple

w3lib/url.py

@@ -264,3 +264,8 @@ def any_to_uri(uri_or_path):
         return path_to_file_uri(uri_or_path)
     u = urlparse(uri_or_path)
     return uri_or_path if u.scheme else path_to_file_uri(uri_or_path)
+
+
+__all__ = ["add_or_replace_parameter", "any_to_uri", "file_uri_to_path",
+           "path_to_file_uri", "safe_download_url", "safe_url_string",
+           "url_query_cleaner"]