Adds Wunderlist agent

The Wunderlist API is currently in beta, so this should be considered a WIP.

Currently you can just add a new task to one of your lists, I am wondering if it would be useful to be able to manipulate tasks or list (mark as completed, delete, etc). The problem is that you need the task/list id (or the exact name so the agent can look up the id) which would probably be a bit complicated to with the agents we currently have.
Thanks, will add it tomorrow. The latter is not really needed because you can not create an agent without a service, maybe we should use the `gem_dependency_check` before propagating an event or run the check function to avoid failed jobs. On the other hand those failed jobs might help detecting what is going wrong.

I think we also should add some notes to the services page to give the user a clue, why it is not possible to authenticate again a given service.