Bug in LKTracker.

There is a bug which needs to be fixed in LKTracker. https://github.com/jayrambhia/SimpleCV/blob/develop/SimpleCV/Tracking/LKTracker.py#L142

```
if not dx or not dy:
    track = LK(img, bb, new_pts)
    return track
```

This should be `LKTrack(img, bb, new_pts)`
