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Two changes are included in this pull request:

1. **Allow a member authority to send a credential back to the originating tool.**
  If an MA has stored credential that matches the user and tool URNs, allow the MA to send that back without requiring that the user create a new credential.
2. **Allow an originating tool to specify a default member authority.**
  If a member authority knows which MA a user needs, allow it to specify that to the xml-signer to streamline the user experience. This also helps for development where a hidden MA can be appended to the list.