Multiple turrets exploit and mob sprite updates fix.

Removed the sleep(10) on the New() of the turrets. This will stop the exploit of making several turrets by spamming the last step on the turret construction.

The cover of the turret will also spawn at the New() of the turrets, instead of waiting for the next process() tick.
Fixes all the lack of sprite updates while constructing a turret and added the "you need two sheets of metal" message that Rolan7 made on his pull request. (which I'm closing muahaha)