wunderlist: update_task_due_date and update_task_title methods

What are your thoughts on extending the API available from the Wunderlist ApiClient object? 

For my project, triaging tasks (by renaming their title or updating their due date), I needed access to these parts particularly.

I imagine there's a more general approach to this, but haven't yet investigated. This was just a bare minimum to get things working for me.

(Thanks for the great project!)