reactions to first feedbacks from thaljef
adding a local TODO to track things... to do
Adaptations after feedback from thaljef

Getting rid of multiple-stacks exports
Moving logic for checking inside the Action class
Adapted documentation for single-stack export
Added more to-do's
initial adaptation to single-stack export
Most suggestions from thaljef

* moved to single-stack export
* default filename as STACK-UUIDPREFIX.EXTENSION
* support for tar.gz and tar.bz2
* most validation logic moved in Action class
* hardlinking in directory generation (when possible)