Limit bullet count

[gdamore]

Right now players have unlimited shots. That makes the game kind of too easy. We need to have some kind of limiter on this. That also means that we need to have a way to register a "watch" on the bullet objects to keep count of them. Alternatively we could just keep track of the last X fired shots, and limit to not more than X bullets per time interval.

The former approach would allow us to create more bullets when they are destroyed without waiting. The latter would not -- our fire rate would be fixed.

A third approach and simpler still, would be to inject a minimum delay between shots. Sort of a reload time. This is probably the most accurate for simulation, but I'm not sure it gives the best play results.