[CONFIGURATION-534] Slightly reworked handling of include files in PropertiesConfiguration.

Include files are now enabled per default, even if no base path is set.
Get and set methods for the includesAllowed property are now public and
complient to JavaBeans conventions.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1457498 13f79535-47bb-0310-9956-ffa450edef68