Factory create method - object literal Entity descripton:
{
   position: {x:0,y:0,z:0},    // used as the start position for particles - default (0,0,0)
   positionRnd: {x:0,y:0,z:0}, // randomness to apply to the start position - default (0,0,0)
   rate: Number,               // particles per second to emit - default 0
   maximum: Number,            // maximum allowed particles (zero for unlimited) - default 1000
   velocity: {x:0,y:1,z:0},    // start velocity of the particle - default (0,1,0)
   velocityRnd: {x:0,y:0,z:0}, // randomness to apply to the velocity - default (0,0,0)
   lifetime: Number,           // lifetime in ms of the particle (zero for unlimited) - default 0
   lifetimeRnd: Number,        // lifetime randomness to apply - default 0
   gravity: boolean            // true to apply gravity to particles - default true
   style: {...}                // particle rendering style (@see Phoria.Entity)
   onParticle: function() {...}// particle create callback function
}