in single_stream_class.py: bbox y coordinate wrong 

In single_stream_class.py, line 156, I think the minus sign is wrong:
```python
bounding_box.center.y = float(top - (height/2))
```
should be
```python
bounding_box.center.y = float(top + (height/2))
```
Please, correct me, if I'm wrong. 
Otherwise the package works fine for me.
Thanks for your awesome work!