Add unit test class for analyse_commons_dump.py

This legacy code could really use some unit tests
before trying any refactoring.
Add test for get_categories_from_text

This method parses wikitext looking for categories using regexes.
Add simple test checking no results, one result and two results.
Add test for timestamp_to_date.

The method converts a MediaWiki timestamp to a datetime.date object.

Add straightforward unit test for it.
Merge branch 'master' of github.com:Commonists/wm_metrics