- I've added a backend for Copy.com cloud storage, this supports all the
    required operations and works as it should from my tests.
  - You can use it by calling duplicity with something like:
    copy://account@email.com:your-password@copy.com/duplicity
  - The only thing I've concerns with is the optimized support for _delete_list
    which can't be enabled here because the test_delete_list tries also to
    delete a not-existing files, and it requires the backend not to raise an
    exception in that case (is this somewhat wanted or could we do the same as
    for _delete or _query?)