Simplified sorting of directory listing
Some visitor behavior and API cleanup
Added test to prove parallel listing & downloading
Ranamed zip-package and logging visitors
Extracted file-service enumerators

Achieved a state, where both enumeration of a source file-system and the
way output is stored via visitor are totally abstract (check C# semi
Visitor Pattern). This should allow, with minimal effort, to implement such
a futuristic features like:
 * enumerating:
   - target device file-system
   - local file-system
   - ZIP package
 * storing:
   - in a local file-system
   - memory-buffer (on fly preview functionality?)
   - ZIP package

And then mixing them as needed.
Added copy-to-local and target-preview features

Created basic support for downloading files from target device directly
into a local file system (no compression involved) and into memory
buffers. The second should nicely help to build a 'file-preview'
functionality.