prevent petra to build siege towers as it does not know how to use them

git-svn-id: http://svn.wildfiregames.com/public/ps/trunk@15918 3db68df2-c116-0410-a063-a993310a9797
Upgraded persian buildings:
blacksmith, dock, farmstead, defense tower, wonder


git-svn-id: http://svn.wildfiregames.com/public/ps/trunk@15919 3db68df2-c116-0410-a063-a993310a9797
Merge 'remotes/trunk'