Fix for missing helper module

Hello, `helper.py` was missing out.

```
Sep 08 19:34:15 xyz manager.py[11979]: Traceback (most recent call last):
Sep 08 19:34:15 xyz manager.py[11979]: File "/usr/lib/zfs-snap-manager/manager.py", line 33, in <module>
Sep 08 19:34:15 xyz manager.py[11979]: from zfs import ZFS
Sep 08 19:34:15 xyz manager.py[11979]: File "/usr/lib/zfs-snap-manager/zfs.py", line 25, in <module>
Sep 08 19:34:15 xyz manager.py[11979]: from helper import Helper
Sep 08 19:34:15 xyz manager.py[11979]: ImportError: No module named helper
```