Add the conftest.py file

This file was added to simplify how the tests can be run, from a
developer's point of view (not have to set the `PYTHONPATH` explicitly
before running the tests).

This decision can be reevaluated once the project has a `setup.py` file.

Author: diogoosorio

tests/conftest.py

@@ -0,0 +1,4 @@
+import os
+import sys
+
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..'))