Added a method for checking whether a FileLocator points to a file.

This is certainly a useful information. It is also required by FileHandler's
load() and save() methods to verify that sufficient information is available.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1523797 13f79535-47bb-0310-9956-ffa450edef68