Add tox config.

tox.ini

+# Tox (http://tox.testrun.org/) is a tool for running tests
+# in multiple virtualenvs. This configuration file will run the
+# test suite on all supported python versions. To use it, "pip install tox"
+# and then run "tox" from this directory.
+
+[tox]
+envlist =
+    py26,
+    py27,
+    py33,
+    py34,
+    doc,
+
+[testenv]
+commands = {envbindir}/py.test {posargs}
+deps =
+    pytest
+    lxml
+
+[testenv:doc]
+changedir = doc
+setenv = APIDOC=True
+
+deps =
+    sphinx
+commands =
+    sphinx-apidoc -f -T -M -o api {toxinidir}/et_xmlfile {toxinidir}/et_xmlfile/tests
+    sphinx-build -b html -d {envtmpdir}/doctrees . {envtmpdir}/html