Incremental write

Hi Chris and others,

I have implemented the first version of incremental/buffered/lazy writes.
The implementation includes tests, and it seems to work properly.
However, I am not convinced that the way to augment the present functionality is the best one,
although it's 100% not colliding with existing code. 
The possibility to rename files via add_data is also a feature I need.
Today I use an Archive::Tar::Streamed like workaround in production, which has shown some problems with files > 2 GB;
before re-inventing the wheel in too many places, I thought fixing the original would be a more efficient strategy :-).
It would be great if you could consider merging in or using my changes, since I would need to have some kind of stable result ...

Best regards,
    Markus

P.S.: I also marked a place where I think that following symlinks is done far too late.