Add tox support.

tox.ini

@@ -0,0 +1,23 @@
+[tox]
+envlist = py26,py27,py33,py34,py35,py36,pycodestyle
+
+[testenv]
+deps = pytest
+commands = pytest
+
+[testenv:pycodestyle]
+deps = pycodestyle
+commands = pycodestyle ./stix2
+
+[pycodestyle]
+ignore=
+max-line-length=160
+
+[travis]
+python =
+  2.6: py26
+  2.7: py27, pycodestyle
+  3.3: py33
+  3.4: py34
+  3.5: py35
+  3.6: py36