stem parse_file errors when using descriptor_type='server-descriptor 1.0'
On a call like "stem.descriptor.parse_file(f,'server-descriptor 1.0')" the new parse_file() fails with an error like "ValueError: Line contains invalid characters: @downloaded-at 2012-09-17 17:44:50" or "ValueError: Line contains invalid characters: @type server-descriptor 1.0" when it encounters annotations in a descriptor file, and it fails with an error like "ValueError: The 'router' entry can only appear once in a descriptor" when there are no annotations but multiple router entries.