Made type parameter of configuration builder classes more flexible.

The result class managed by the builder does not have to be of type T, but can
extend it.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1592016 13f79535-47bb-0310-9956-ffa450edef68
Adapted test class to verify that the more flexible type parameters work.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1592017 13f79535-47bb-0310-9956-ffa450edef68