SecretService cleanup

[jaraco]

In this comment, Jonathan makes some good suggestions. Let's implement them.

• Refactor the DBus initialization code to remove duplication.
  

• Test a little bit more the supported() method. The existing implementation just checks one 'unsupported' case, and it's not even trivial by reading the test that it should be unsupported
  

• Beware of BackendBasicTestCase, you _have_ to overload the supported() method otherwise most of the useful tests will be completely skipped!
  

• No need to return None in get_password()
  

• Document the calls to 'assert prompt == "/"' (comment/assert message) so that we know what it is used for and if it failed, why is it important that it failed. 
  

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• Bitbucket: https://bitbucket.org/kang/python-keyring-lib/issue/61
• Originally reported by: Jason R. Coombs
• Originally created at: 2012-04-09T14:00:38.292

[jaraco]

It appears the third item is no longer relevant. I've implemented the other four.

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Original comment by: Jason R. Coombs