Particle Effects

Added particle effects to the game.

To create a particle system, objects pass the name of a particle_system
to the ParticleSystemsHandler, along with the duration of the effect and
either a position or a SceneNode to attach the system to.

The ParticleSystemsHandler is responsible for creating the particle
systems, stopping their emitters after the specified time passes, then
destroying the system when all it's remaining particles expire.

Currently, all particle systems are lost on loading a save game and time
passed while paused counts against system duration.