Add mac_unix_expanded format

Most of the utilities use zero-filled representations
for mac address on *nix systems. The existing mac_unix
formatting strategy, however, crops leading zeroes which
may cause different kinds of problems when integrating
several services or software.

This patch adds a mac_unix_expanded formatting strategy which
allows to represent mac addresses as zeros-filled strings.
Merge pull request #78 from romcheg/mac_unix_unstripped

I also like "expanded" better.

@drkjam: I'd also like to include ticket #73 in the new release. We also need to update the Changelog and the EUI database. We should also remove the coverage_report.txt from the repo.