Implemented methods for finding the source for a given key.

Added a new method getSources() to CombinedConfiguration which retrieves all
configuration sources that contain a given key. The existing getSource()
method is now working again.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1587019 13f79535-47bb-0310-9956-ffa450edef68
Fixed remaining failing test cases for CombinedConfiguration.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1587020 13f79535-47bb-0310-9956-ffa450edef68