New Clover Mechanic!

Added a new mechanic called the Lucky Clovers!  You get three lucky clovers when you start a new game, and you can use them to duplicate any tower you currently have built.  You only get three for the whole game, so save them until you really need them!  

I talked briefly in my previous devlog about this, the goal of the clovers is (hopefully) to give players an action they can take if they get stuck on a wave besides reloading to an earlier save.  Because the clovers are there from the start, hopefully they feel like scarce enough resource that they are not used right away, but I guess we shall have to see!