Add additional credentials when there is no explicit credential
Log authentication attempts to get a chance to find out the realm.
Fix issue with Subversion Tagging plugin failure.

Now, just make sure you've configured an 'additional credential' for
the repo to tag, and it works.
Simplied code as null check is redundant
Fixed NPE
Merge pull request #70 from daniel-beck/notify-commit-external-credentials

Fix credentials in some use cases