Add user environment customization class

Users probably want to add some specific content to their profile.
An extra class containing all customizations seems like the most
flexible solution for this.
Add dependency between user and env_class
Merge pull request #21 from bkkrw/master

Add user environment customization class
Add owner and group parameters to custom environment class
Documentation string for the new class parameters
Merge pull request #22 from bkkrw/master

Add owner and group parameters to custom environment class