Preserve executability of files for tar archives

[masc3d]

contributing to #23

[thrau]

it seems that this PR only covers one very specific case and introduces quite a lot of changes just to make that one case work. i think it would be more useful to take a more general approach to #23 using the Java 7 features.

[masc3d]

@thrau yes, it covers "only" the most common and relevant use case, as preserving executability is a key feature of eg tar archives.

there's not really many changes here, but you probably refer to fixing the inconsistent way factory has been handling CommonsArchiver
This was necessary and should be merged anyhow as it's a potential generic defect.

imho best way forward would be to merge and do minor cleanup if necessary after #23 has been implemented.

[masc3d]

everything to make it work is essentially in specialized TarAchiver