Fixed build for MacOS + added rate limiting for game simulation

The boost libraries needed to be added to the makefile for the MacOS settings

Also, it ran way too fast on my laptop, so I've slowed the simulation down by making the increments dependent on the delta time.

For example, if each frame is **1s** the an alien might move

     alien.X += 0.1 * delta // where delta is 1

but if each frame is only **0.5** seconds, then the simulation will slow down accordingly

     alien.X += 0.1 * delta // where delta is now 0.5

hence, regardless of how often the frames render, the simulation will move along at the same 'real' speed

The timings seem a bit funky, in particular, the aliens move down the screen very fast, but I assume you can tweak that.

Also, I've re-added the .gitignore file