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.
I'd suggest also adding something like:

```ruby
#{'## Include the `omniauth-wunderlist` gem in your `Gemfile` and set `WUNDERLIST_OAUTH_KEY` and `WUNDERLIST_OAUTH_SECRET` in your environment to use this Agent' if dependencies_missing?}
```

and also add

```ruby
gem_dependency_check { Devise.omniauth_providers.include?(:wunderlist) }
```