standardize create_categories.py

- CreateCategoriesBot extends pywikibot.Bot and uses self.userPut()
  with built-in 'always' option

- call pywikibot.showHelp() in case of insufficient arguments

Change-Id: I1417cbc85c8c5d0e9105d23a577d1d79b6074802
Merge "standardize create_categories.py"