cambrionix import error

This looks looks like it might be a circular import problem:

```pytb
$ python -c 'import openhtf.plugs.cambrionix'
Traceback (most recent call last):
File "", line 1, in
File "openhtf/plugs/cambrionix/__init__.py", line 17, in
from openhtf.plugs.usb import local_usb
File "openhtf/plugs/usb/__init__.py", line 39, in
from openhtf.plugs import cambrionix
ImportError: cannot import name cambrionix
```