Added first step towards tournament mode: rankings can now be calculated
on the fly by tag. Tags can be tables, events, butterflies and zebras 
and moonbeams and fairy tales. 
Missing: 
- currently no UI to add tags to a game, except table
Known issues:
- link from tag filtered rankings to player details shows overall player
details
Merge pull request #65 from squix78/master

Added first step towards tournament mode: rankings can now be calculated