Allow importing of properties files

Provide the capability to reference another properties files inside of the one being loaded. There are various levels of complexity that could be implemented here, such as what to do if the file is not found and how the import is achieved (bring the properties into the core manager or create a new manager that is referenced).

A good starting point may be to look at the {E}Properties project (http://code.google.com/p/eproperties/). It may be worth matching syntax to provide compatibility with what they have created.