Update copyright headers to `20XX-2013` instead of just `2013`

Quite some of the ai code seems to exist since 2008, which probably are
issues with either our git-svn migration or `git log --follow` detecting
false positives here. The (very) hackish script used is attached below.
Do we want to manually update some of those headers to be more accurate?
```bash
for f in $(find . -name "*.py")
do
	fy=$(git log --date=short --follow --pretty="%ad" -- "$f" \
		|tail -n1 |cut -d- -f1
	sed -i -e "s@\(Copyright (C) \)2013\( The Unknown Horizons Team\)@\1$fy-2013\2@g" $f
done
```