Adding duration property for Item model.

[AhmedZaki99]

The new duration feature was announced in July this year, and since then, tasks returned by the API include a duration property for paid users like this:

"duration": {
    "amount": 15,
    "unit": "minute"
}

I think it's easy to support (correct me if I'm wrong). If we add a new property in the Item model then JSON mapping will probably do the rest. Similar to the DueDate object.

[AhmedZaki99]

I've made the proposed change and tested it on my fork, and it's working fine both ways (read and write).

I will create a pull request for it and link it to this issue.