tox.ini: configure tox test environment; set up py.test to discover both doctest and unittest tests

tox.ini

@@ -0,0 +1,21 @@
+[tox]
+envlist = py27, pypy, py33, py34, py35
+
+[testenv]
+deps =
+    pytest
+    -rrequirements.txt
+commands =
+    py.test Lib/fontTools
+
+[pytest]
+addopts =
+    # run py.test in verbose mode
+    -v
+    # show extra test summary info
+    -r a
+    # run doctests in all .py modules
+    --doctest-modules
+    # py.test raises ImportError with inspect.py (requires pygtk) and with
+    # reportLabPen.py (reportlab). They don't have doctests, it's OK to skip.
+    --doctest-ignore-import-errors