'disk' emitter, radiusSpread, angles

* Included disk-shaped emitter
* now able to randomize radius size for sphere/disk emitter
* particles can have an angle, angle can be randomized or aligned to velocity direction
* fixed _resetParticle velocity direction issue
* TODO: incorporate angleVelocity for rotating particles
Merge pull request #8 from stemkoski/patch-1

'disk' emitter, radiusSpread, angles