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: NLucas Meneghel Rodrigues <lmr@redhat.com>

avocado/core/tree.py

@@ -13,9 +13,23 @@
 #
 # Authors: Ruda Moura <rmoura@redhat.com>
 #          Lucas Meneghel Rodrigues <lmr@redhat.com>
+#          Jaime Huerta-Cepas <jhcepas@gmail.com>
+#
 
 """
 Tree data strucure with nodes.
+
+This tree structure (Tree drawing code) was inspired in the base tree data
+structure of the ETE 2 project:
+
+http://pythonhosted.org/ete2/
+
+A library for analysis of phylogenetics trees.
+
+Explicit permission has been given by the copyright owner of ETE 2
+Jaime Huerta-Cepas <jhcepas@gmail.com> to take ideas/use snippets from his
+original base tree code and re-license under GPLv2+, given that GPLv3 and GPLv2
+(used in some avocado files) are incompatible.
 """
 
 import collections