Changed group to be "$SALVE_USER_PRIMARY_GROUP"

This is in place of the special behavior when group is unassigned.
Although it introduces an extra environment variable, it is a reduction
in mysterious special behaviors.
Updated README to 0.2.0

Added README references to $SALVE_USER_PRIMARY_GROUP
This includes removal of comments about special behaviors on unspecified
attributes.
Merge branch 'feature/group-env-var' into dev