PLUS : Initial version of upgradeable towers

* You can upgrade the range and the attack speed of towers
PLUS : Fully functional tower update mechanic

* you can now upgrade up to 3 times, whatever you want, but to a total
of 3 times.
* the more you upgrade, the more the new thing will cost.
BUG FIX : Upgrade tower without money

* You would be able to upgrade a tower and get a negative amount of
resources, because I forgot to put another IF in
* added a texture for the sniper/sharpshooter
* every upgrade now grants 1/8 range or 1/8 attackspeed.