Manage users (and a few other tweaks)

1: Derive package name from version
2: Added the facility to manage both the user and the group that owns the Nexus installation and work directories. This solves issues where these directories are owned by some other user (often root) and Nexus fails to start due to permission restrictions.
3: Changed a few of the task names to make them describe what they do slightly more clearly
4: Added spaces between variable names and braces to let them breathe (and be more idiomatic)