init ParticleSystem with particles already part way through their life spans

in response to [this tutorial on the Starling wiki](http://wiki.starling-framework.org/tutorials/create_stars_background) and this [pull request](https://github.com/PrimaryFeather/Starling-Extension-Particle-System/pull/10), i propose adding just one method to ParticleSystem.

doesn't require exposing any members or any significant overhaul, just starts up the ParticleSystem with a specified number of Particles already part way through their life spans.