17228 spent time is not part of wp in api v3

[``* `#17228` Spent time not part of API V3's work package response``](https://community.openproject.org/work_packages/17228)

:exclamation: Remove ``model`` when merging this into dev branch.
I already thought about adding a small type definition for such durations in the **Basic Objects** section of the spec, because I found `estimated_time` quite unclear:

- which values are allowed for `unit`?
- should the unit be singular or plural?
- why does the implementation return something like `{ "unit": "hours", "value": null }`?
- etc

Now that I think about it... probably it was a good idea if someone came up with some kind of standard that everyone would understand right? Some standard defining how to represent dates, times and durations... Wouldn't that be great?

...

[**ISO 8601** also defines durations](http://en.wikipedia.org/wiki/ISO_8601#Durations), which are also used in [XML schema](http://www.w3.org/TR/xmlschema-2/#duration). Probably it is a good idea to stick with a standard rather than defining our own one.

Always remember:
![XKCD on ISO 8601](http://imgs.xkcd.com/comics/iso_8601.png)