make the install.rdf well-formed

Hi, I am packaging this for [Debian](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=736844). As part of the build process we parse the install.rdf and check it for errors. The current file is not well-formed:

{{{
Traceback (most recent call last):
  File "/usr/bin/install-xpi", line 289, in <module>
    main()
  File "/usr/bin/install-xpi", line 286, in main
    options.system_prefs, debian_directory, options.verbose)
  File "/usr/bin/install-xpi", line 214, in install_xpi
    extension_id = get_extension_id(os.path.join(copy_dir, "install.rdf"))
  File "/usr/bin/install-xpi", line 84, in get_extension_id
    "SELECT ?id WHERE {?x em:targetApplication [] . ?x em:id ?id }"))
  File "/usr/bin/install-xpi", line 69, in get_query_field_id_as_list
    parser.parse_into_model(model, "file:" + rdf_path)
  File "/usr/lib/python2.7/dist-packages/RDF.py", line 1801, in parse_into_model
    raise err
RDF.RedlandError: "property element 'localized' has multiple object node elements, skipping."
}}}

This is because each `<Description>` element should have its own `<localized>` parent element.