Fix indention of tree implementation

Just fixed some indention differences in the module implementing the
tree data structure.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Add pre-, in- and postorder traversal

I added the recursive variants of the preorder, inorder and postorder
traversal algorithms. The iterative variants would be more interesting.
I will add them later.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>