Added support for testing with tox.

This works fine with the caveat that setuptools-git must be installed or
it will fail to copy the REGISTRY.txt over which is required to bundle
the sdist.

tox.ini

@@ -0,0 +1,14 @@
+[tox]
+envlist = 
+    py25,py26,py27,py32,jython,pypy
+
+[testenv]
+commands = 
+    python setup.py test -q
+deps =
+    virtualenv
+
+# we separate coverage into its own testenv because a) "last run wins" wrt
+# cobertura jenkins reporting and b) pypy and jython can't handle any
+# combination of versions of coverage and nosexcover that i can find.
+