There is no need for introducing unit velocity concept to the engine now, there is already much easier and driect way to know whether a unit is moving or not
How:
Introduce a new Object Attribute inside EntityObjectAttribute enum 
OBJATTR_IsMoving
Handle it inside StarCraftEntity::Attr as follows
```
case OBJATTR_IsMoving:
    return m_unit->isMoving() ? 0 : 1;
```

See method documentation in BWAPI here: https://code.google.com/p/bwapi/wiki/Unit#isMoving