Return raw values in `_login_with_link` to able to process them further.

[exislow]

We have currently no method, which allows to handle an OAuth (device login) by another applications thread pool.

I suggest to use login_oauth to return LinkLogin and the future instance. _login_with_link will only return the JSON values and can be used to bootstrap the login in 3rd party apps code and their own thread pools.

I will get you a PR.

[exislow]

There you go: #303

Let me know what you think.

[tehkillerbee]

Fixed in #303