fix(group): Add core to the wheel group so it can use su

Without pam, su enforces the use of the wheel group by default.
fix(sudoers): Switch to using an alternate sudoers location.

Instead of dropping a link into /etc/sudoers.d change the main sudoers
file location from /etc/sudoers to a location under /usr. That way the
default configuration is safe but still allow custom rules in sudoers.d.
Merge pull request #10 from marineam/sudo

Sudo