FIX: Add mp4 mime type.
FIX: Inheriting supplemental groups from root [issue 329]

The new policy is:
1. If UserAccount and GroupAccount are defined, set the UID and GID and restrict the supplemental groups to only that GID. This means you get one UID and one GID. No supplemental groups.
2. If UserAccount is set and NOT GroupAccount, then call initgroups to restrict the groups to those that the user is a member of including the group mentioned in /etc/passwd
FIX: add mp4 to mime.types.
DOC: Update GroupAccount and UserAccount doc.