Add Swedish localization

Greetings! I just found out about todonotes and noticed my native language was missing from the list of localizations. Thought I'd do something about that. :D

Feel free to review and criticize my changes. I did my best to make all changes necessary, but I'm not completely sure. In particular the comment on line 1114 of 44135cbe84d56c7aadd350dbf192e2d49d3f0ca4 is a bit confusing, but it looks like a relic from days when there weren't as many localizations.