config file overlays

e.g. default to grouper.properties, and then if there is a linked file to another grouper.properties (or another external config file on server), then overlay. This could be for the following properties files:

grouper.properties
grouper.hibernate.properties
grouper-loader.properties
grouper.client.properties (in grouper)
grouper-ws.properties

for the sources.xml, we could just specify the location of that in the grouper.properties, and if not specified, defaults to the classpath as before (wouldnt be an overlay since that would be more complicates)