Module 5: FTree is incorrect

[xgillard]

**This issue relates to #35 ** (should probably be a subtask of it).

The FTree expects a depth of 0 for leaves. This is not correct as a leaf is a (sub)tree comprising one single node. Therefore, it has a depth 1.

Also, rather than relying on left() == null && right() == null it would be better if all nodes implemented an isLeaf() method. That way, leaves could be a Tree implementation totally separated from the Node class. This would be way more functional (akin to the Nil in functional lists).