Make sure password from test_delete_one_in_group is properly cleaned up.
document platform_ rename in README

the util.platform module was renamed in b2629a6ef04f
due to a problem in #112
SecretService: use a different function to check that the backend is functional.

The default collection may not exist, but the collection will remain usable in
that case (and will be able to create default collection on first use).

Also, make the error message more verbose.

Resolves https://bugs.launchpad.net/bugs/1242412.
Bumped to 3.2.1 in preparation for next release.