2.0.0 (not released yet)

The library completely redesigned, tons of refactorings applied.

Movement methods now use ActiveRecord::Base#save so all user validations and callbacks are invoked now.
parent_id and position attributes are no longer protected by default. It's up to developer to make them attr_protected now.

Reduced amount of UPDATE SQL-queries during movements and reorders.
Added collection iterators:
1. each_with_level which yields each node of collection with its level.
2. each_without_orphans which yields only that nodes which are not orphaned (i.e. that nodes which don't have corresponding parent within iterated collection).
3. arrange which constructs hash of hashes where each key is node and value is its hash of its children.
Added ability to explicitly set left/right sibling for node via #left_sibling= and #right_sibling= methods (#30).
.leaves scope now works even with models that don't have counter_cache column (#29).
Added method instance method #move_to_child_with_position which is similar to #move_to_child_with_index but is more human readable.
Full support for before_add, after_add, before_remove and after_remove callbacks (#25).
Descendants now can be arranged into hash with #arrange method (#22).
Flexible control over tree traversals via blocks passed to #descendants and #ancestors (#21).

Deprecated methods:
1. #insert_at. It is recommended to use #move_to_child_with_position instead.
2. #branch? is deprecated in favour of #has_children?`.
3. #child? is deprecated in favour of #has_parent?.
4. #move_possible? is deprecated in favour of built-in and user defined validations.

Bug fixes:

Fixed several issues that broke tree integrity.
Fixed bug when two root nodes could be created with same position (#24).
Fixed support of STI and basic inheritance.
Fixed bug when user validations and callbacks weren't invoked on movements via move_to_* methods (#23).

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