re: loadStrings & loadJSON for preload, using Grunt this time

Everything here is the same as my last Pull Request except that I modified the src files this time and used Grunt to compile/test.

I tweaked loadStrings and loadJSON, and also modified the way that core.js keeps track of the preload count. Before I made this change, my tests would sometimes get stuck on preload() and not move ahead to setup(), especially when I did non-preload functions in preload, like assigning variables.