add Tox configuration for tests

tox.ini

+[testenv]
+deps=
+  nose
+  coverage
+  mox
+commands=
+  nosetests \
+	--with-coverage --cover-package=http_hmac --cover-erase --cover-html --cover-html-dir=htmlcov \
+	--with-xunit \
+        []        # substitute with tox' positional arguments
+