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.
That's a good idea.  Nice to list the possible services, and which can be used.  But since you can also try to new-up an agent, it's also nice to show a message if it cannot get a service in the description.