added options and path collisions

I am finding groups, singletons, and copy node path to be very helpful.

Created a singleton to hold global variables

I added options menu with
sound on off
music on off

on map play:
I added placement collisions so towers cannot be placed on roads or enemies or objects or gui objects.
added lives gui and countdown when marbles escape
added starting gold and gold earned each marble destroyed

Infrastructure is nearly done. Then it will be just the grind of adding new towers, enemies and levels.