[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()
Squash
[feature/nestedsets] Implement move_children()
[feature/nestedsets] Wrap move_children() with a lock

This should avoid problems with multiple processes running at the same time
[feature/nestedsets] Add unit tests for move_children()
[feature/nestedsets] Use setUp() instead of repeating
[feature/nestedsets] Split of a base for the tests
[feature/nestedsets] Readd rest of the tests
[feature/nestedsets] Make get_sql_where public so it's easier testable
[feature/nestedsets] Add require for test's base file
[feature/nestedsets] Add unit tests for set_parent()
[feature/nestedsets] Implement set_parent()