Rename 'isValid' static method. Fixes #93.

Rename 'isValid' method to 'isValidNestedSet' to avoid name clashes with validator packages.
Add 'force' option to rebuild operations.

This will force the index rebuilding process even if the underlying nested set structure is valid.
Change 'makeRoot' implementation.

Moving to a root node now promotes the provided instance to be the rightmost root in the tree, thus being more consistent. We are no longer moving the current subtree to the right of the first available root.
Implement trunks family of methods. Closes #59.

Implement 'allTrunks', 'trunks' and 'isTrunk' methods to complement the leaf and root ones. We consider a trunk a node which falls in the middle of a branch (not root and not leaf).
Remove unneeded check on soft deletes test.
Touch all nodes involved in a move operation.

Because of a previous impl change, the nodes involved in a move operation would not get 'touched'. Baum will now update the 'updated_at' field manually if it is used in the base model.
Update CHANGELOG & bump version.