Fix node removal algorithm for the AVL tree

[errendir]

The code responsible for removing nodes from the AVL tree can leave the tree in an invalid state. Whether a node is a left or a right child of its parent is not correctly detected. This is due to a typo. Compare:

this._compare(val, current.getParent().getValue() < 0)

with

this._compare(val, current.getParent().getValue()) < 0

The first line is not a correct comparison, since the expression current.getParent().getValue() < 0 is a boolean which should not be compared with val which may be a string or a number. The intention is to check which value (either val or current.getParent().getValue()) is larger, so this._compare should be applied to those directly and then compared with 0. This PR fixes this issue.

Additionally the .remove method of AvlTree sometimes doesn't do enough rebalancing. I added a test which shows when that happens and I also added a fix. The problem was with the "case 4" of the removeRecursively function not calling this._balanceNode