Resource property source

Hi,

This pull request adds a new `util.PropertySource` implementation based on resources provided by the default class loader.

This is somewhat related to Olli's pull request #179.

The functionality provided by this pull request will be used later to allow runners (scriptlet, command, etc) to read configuration from a xar archive using the class loader to do the work.

As a use case: you have a project (containing a `util.cmd.Command`) that is packed as a xar archive containing both classes and configuration. In order to run it, you will run:

```sh
$ xpcli -cp /path/to/project.xar -c res://conf/dev com.project.cmd.DoSomethingCommand
```

where inside the `project.xar` archive, there are packed configuration files like `xar:///conf/dev/log.ini`, `xar:///conf/dev/database.ini`


Let me know what you guys think.

Cheers,
-- Marius