avocado.core.tree: Add copyright note

Attributing copyright to Jaime Huerta-Cepas <jhcepas@gmail.com>,
author of the original tree node structure, that we drew inspiration
from and used part of the code (tree drawing).

CC: Jaime Huerta-Cepas <jhcepas@gmail.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>