expose app.dir

Application that want to access embedded resources need to know the path to expanded zip. Most developer would expect the JVM to be started from this directory and just use
```java
   new File("./myResource.txt")
```
but for whatever reason the JVM is started from genapp directory, and "/app" need to be added.
This pull-request exposes a app.dir system property for developer to avoid hard-coding this path.