added tox.ini and used Tox to verify compatibility for py27 py32 py33 pypy

tox.ini

@@ -0,0 +1,13 @@
+# 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 = py27, py32, py33, pypy
+
+[testenv]
+deps = 
+    nose
+commands =
+    nosetests