Now amount of parallel streams can be defined per input and output device
Setup statistics structure, which for now is passed to the ReadController only
Implemented statistical counting for writer and the hashers.
Initial version of quite usable stats

next up is delta tracking, and of course amount of files done so far
Added total read and written counters, improved printing style
Delta-printing now looks real good !

Also, it's amazing to see how fast it is !
Greatly improved looks of statistical information
Improved looks of progress and summary printing even further
Added ability to shut down logger go routine.

Nonetheless, verify after seal still yields wrong stat resutls
Of course, it works flawlessly if run separately, and in every other way ...
what's going on ??
Fixed incorrect handling of values/pointers

Previously commands where treated as value types, which was valid as
long they didn't carry the stats structure.
Now that they do, NewCommand must return a heap-allocated object, otherwise
readers and writers point to the wrong memory location after returning it.
An alternative would have been to make the stats field a pointer.
Improved verify input checking, and fixed progress logging format
Added LogLevel member which allows log filtering
No progress printing unless log level is at least Progress
Now the log-level is parsed from the commandline and passed down to the ones who care.

It's a trivial implementation, and what I'd be missing is a way to have progress logging independently
form the logging verbosity. However, I believe in most cases, people won't bother anyway.
Even I don't see a real application for this feature ... .
Considering it added some changes which aren't so nice, maybe it shouldn't be there in the first place ?
Merge branch 'stats'