Added operator overloading to pf::MAC so that MACs can be compared
regardless of how the MAC was passed to the constructor.

E.g. 00:00:de:ad:be:ef eq 00:00:DE:AD:BE:EF should return true.
This differs from the behavior of Net::MAC.

Added an "in_OUI" method for convenience.
Allows to check if a MAC is part of an OUI regardless of case or
delimiter.
Modified get_oui to return the format expected by PF and added a
get_IEEE_oui method.
Merge pull request #153 from louismunro/feature/Class_MAC

Feature/class mac