add tox.ini configuration file for running pytest on Python 2.7 and 3.5

tox.ini

@@ -0,0 +1,25 @@
+[tox]
+envlist = py27, py35
+
+[testenv]
+deps =
+    pytest
+    -rrequirements.txt
+commands =
+    py.test {posargs}
+
+[pytest]
+minversion = 2.8
+testpaths =
+    Lib/ufo2ft
+python_files =
+    *_test.py
+python_classes =
+    *Test
+addopts =
+    # run py.test in verbose mode
+    -v
+    # show extra test summary info
+    -r a
+    # run doctests in all .py modules
+    --doctest-modules