v4.1.0-beta

• #75: Converted to trait. Following methods were renamed:
• appendTo to appendToNode
• prependTo to prependToNode
• insertBefore to insertBeforeNode
• insertAfter to insertAfterNode
• getNext to getNextNode
• getPrev to getPrevNode
• #82: Fixing tree now handles case when nodes pointing to non-existing parent
• The number of missing parent is now returned when using countErrors
• #79: implemented scoping feature
• #81: moving node now makes model dirty before saving it
• #45: descendants is now a relation that can be eagerly loaded
• hasChildren and hasParent are now deprecated. Use has('children')
has('parent') instead
• Default order is no longer applied for siblings(), descendants(),
  prevNodes, nextNodes
• #50: implemented tree rebuilding feature