Change categories handling in post processing

A post processing is likely to return a list of categories,
not only one.

We handle that case in a ugly way, but that should be a non-breaking change.
Add method to compute categorisation statistics.

We want to know how much categories were added as part of
the post-processing.

- During the post processing, we fill two Counters, one
  per category, the other one per file.
- The method categorisation_statistics computes basic fancy stats
  based on these two Counters.
Merge branch 'master' of github.com:JeanFred/MassUploadLibrary