Added new locate() methods based on FileLocator objects.

These methods use a FileLocationStrategy to resolve the passed in locator
objects. They are going to replace the existing locate() variant based on a
base path and file name. This allows hooking into the search algorithm by
providing a custom FileLocationStrategy.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1527229 13f79535-47bb-0310-9956-ffa450edef68