[feature/nestedsets] Add interface and abstract nested sets item class
[feature/nestedsets] Add album implementation of item interface
[feature/nestedsets] Add unit tests for album item
[feature/nestedsets] Add basic interface and class of nested sets
[feature/nestedsets] Implement moving methods
[feature/nestedsets] Implement get_branch_data()
[feature/nestedsets] Implement remove and delete
[feature/nestedsets] Add return values and test them in tests
[feature/nestedsets] Implement get_parent_data()
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