Added a version number constant to accel.h

I also wrote a "gen version number"

This "gen version number" guarantees that x.y.z(alpha)?(beta)? compares correctly, so users can do logic along the lines of (if necessary):

```
ACCEL_VERSION_GEN(1, 2, 3, false, false) < ACCEL_VERSION_GEN(1, 2, 4, false, false)
```