Not change 'left', 'right' and 'tree_id' field on change 'parent'

[petrikoz]

Hi, Mark.

We are using your module in our project: https://github.com/ITCase/pyramid_sacrud_example

We are using decalarative style for SQLAlchemy models. Your module was integrated by instructions from issue #3.

But we have a trouble. Not change 'left', 'right' and 'tree_id' fields when we save data in table with 'parent' field changed.

[miracle2k]

It works when parent_id is changed. When the relationship is changed, the update code does not trigger...