Access parent in workpackage

[c-holtermann]

I'd like to access the parent information in workpackage. I can only do this indirectly through the _links property.

[c-holtermann]

At least it's possible to access that information through the parent href in the _links section which is not the case for projects (#21). It would be nice to have easier access to parent information for workpackages and projects but I guess I'll just have to work around that.