Fix import of get_etherpad_client

Hey good job with this project ! I'm loving it !

This pull request is to fix a problematic import in etherkeeper/core/models.py : get_etherpad_client has moved in your code to etherkeeper/etherpad/views.py

Had to move this function to a new etherkeeper/etherpad/helpers.py to prevent cross import

And this also fix the logout link which had a wrong class defined in the template.

Keep up good work