Multiple improvements

[dimafeng]

1. Register task type. Currently, to extend tasks, you have to use the full name of the task's class:
   task buildClient(type: com.dimafeng.containerizedtasks.NpmContainerizedTask) {..}
2. Configurable node_modules_cache location to allow different source bases be defined within one build script
3. Chown: change owner user/group of the output files to user who runs the script.