Add tox support

Add tox config file, making Jenkins testing easier
(see https://www.shiningpanda-ci.com/docs/tutorials/tox.html
for an example).

More info: http://tox.readthedocs.org/en/latest/example/jenkins.html

tox.ini

@@ -0,0 +1,12 @@
+[tox]
+envlist = py27, py33
+
+[testenv]
+commands = nosetests --with-xcoverage --with-xunit --cover-package=screed --cover-erase
+deps =
+  nosexcover
+  pygr
+
+[testenv:py33]
+deps =
+  nosexcover