Added array of enemies

Rendering still bugged and being tested, but found solution to getting
an array of enemies.

Key is, array elements forget their object type. So an element in
Enemies[] remembers its properties, but not its prototype functions.

Solution is to use function.call(thisContext, args). That is, calling
call() on a function allows defining 'this' in the function call. So
calling
Enemy.prototype.spawn.call(element, x, y, direction);

will cast element to an Enemy type, letting nested calls of this.walk()
or other enemy methods to bind or reference the element correctly.

Adding logging to spawn function to know where enemies are. Could add
more debugging to know under what conditions enemies are rendering, and
correct. Need to get scaling appear correct.