Add resource hierarchy support

[arcady-zherdev]

1. Resource model should be able to have children.
2. A parent resource maxUnits by default is calculated as the sum of its children maxUnits values and cannot be less than that calculated value.
3. A parent resource can be assigned to an event as any regular resource. And when assigned its working time is allocated using the resource calendar as it normally happens for regular resources.
4. A parent allocation is calculated as sum of its won allocation and its child allocations.