Add unittest for parse_xml_dump

This method parses an XML dump to retrieve edits
(a list of triples username/date/file).
Add a simple unit test for it.
Cleanup in analyse_commons_dump.py

Remove useless stuff (unused imports, commented out lines)
Fix method/variables naming from CamelCase
Document methods in analyse_commons_dump.py

Add docstrings to undocumented methods.