Match finished screeps tutorial code exactly

[daboross]

This rewrites the provided code to match, as much as possible, the code in https://github.com/screeps/tutorial-scripts/.

The two exceptions are that I've left in some of the comments, and included code from both the "auto-spawning creeps" and "tower" tutorials, whereas in the tutorial-scripts repository they're separate and never both included at the same time.

Fixes #32.