Add ability to sign pkg files on Mac builds.

@sethvargo Let me know how you want me to fix these issues:

```
spec/unit/packagers/mac_pkg_spec.rb:154:9: C: Align the elements of an array literal if they span more than one line.
        %Q(--distribution "/var/cache/omnibus/pkg-tmp/mac_pkg/Distribution"),
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```

Aligning them as recommended makes the lines really long. I can go back to using previous gsub but I believe this is much cleaner code. But don't care about it too much either way. LMK

/cc: @opscode/client-eng, @opscode/release-engineers