Get the differential between two dates

As a program leader, I want to know how much activity
happened in a collection between two given dates.

Add a method which retrieves the number of revisions,
of edits, and of editors on the collection between two dates.

Add companion method, which calls the above and writes
a human readable text report of it.

This completes our feature set from the old Trutat prototype.
Categorisation metrics on media collection

As a program leader, I want to know the categorisation state
of a media collection.

Add method categorisation_report which iterates over the pages
of the media collection, get the top revision, and collects the
categories in two Counters.

Copy method from the MassUploadLibrary which takes the Counters
and computes fancy metrics on it.
Add module docstring