Add Tree.Sortable extension.

Mix this into any Tree-based class to provide customizable sorting for
nodes.
Tree.Sortable docs.
Move Tree.Sortable history entry to the right version.
Fix lint warnings.
Merge branch 'dev-3.x' into tree-sortable

Conflicts:
	src/tree/HISTORY.md
	src/tree/tests/unit/tree.html
Build tree.
Build yui with new loader metadata for tree-sortable.