application plugin: support running the application from the build directory

With application plugin, you cannot run the generated script right after "gradle startScripts" because the classpath uses 'lib' subdirectory and that directory only exists after the 'installApp' task is run.

This computes 2 classpaths, one using lib, the other using libs and the generated script sets the CLASSPATH variable accordingly at run-time based on the presence (or not) of APP_HOME/libs directory.

Note: the 'windowsStartScript.txt' will have to be adapted for this feature to work on windows machine also, it is just that I don't know how to program correctly BAT files.