avocado.core.tree: Add note about ETE2 and copyright clarification

Add a note in the copyright headers and tree docstrings
recognizing that the base Tree data structure had inspiration
(and the ASCII drawing code) from the ETE2 project, and
hereby list Jaime Huerta Cepas <jhcepas@gmail.com> as one
of the authors of the code, with his express consent.

CC: Jaime Huerta Cepas <jhcepas@gmail.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #160 from lmr/copyright-clarification

avocado.core.tree: Add note about ETE2 and copyright clarification