Dev

Modeled the smoke as noninteracting spherical smoke bubbles (the particles) that fill up with atmospheric air as they grow. When colliding, they are deflected sideways randomly ~~so that the "pressure" doesn't increase~~. EDIT: the proportion of the normal velocity which gets turned to random sideways velocity is ``1 - stickiness``, see last comment.
Added support for drag, buoyancy and weight of said bubbles. You might want to change the defaults.

Note that with drag support, no damping should be used.
Hotrockets will look weird with this (damping, drifting smoke).
I tried making my own .mu file, and while it is okay for testing the general principles, things look pretty ugly.
Note however that the smoke correctly runs along the trench and rises (the drag coefficient and initial velocity of the smoke balloons needs to be tuned, it just looks like there's a giant barbecue underneath the rocket in this screenshot).
![screenshot3](https://f.cloud.github.com/assets/2284290/2405053/d1e6ffbc-aa46-11e3-9ede-8544ef4221f5.png)
Do you have an unity project that I could tweak in order to facilitate debugging?