Initial velocity offset

Implements #5.
```csharp
// The initial velocity of the particles will be offset by a random amount
// lying in a disk perpendicular to the mean initial velocity whose radius
// is randomOffsetMaxRadius. This is similar to Unity's 'Random Velocity'
// Setting, except it will sample the offset from a (normal) disk rather
// than from a cube. Units (SI): m/s.
[Persistent]
public float randomInitalVelocityOffsetMaxRadius = 0.0f;
```
En ce qui concerne l'identificateur, "je n'ai fait celle-ci plus longue que parce que je n'ai pas eu le loisir de la faire plus courte".
Note that while the behaviour of ``fixedEmissions`` has changed slightly internally, its usage is the same to the end user—now correctly documented:
```csharp
// Whether to nudge particles in order to alleviate the dotted smoke effect.
// Set this to true (default) when using 'Simulate World Space' in Unity,
// false otherwise.
[Persistent]
public bool fixedEmissions = true;
```