Replace test runners with nose, and add tox support.

Reviewed in: https://codereview.appspot.com/6490081/
diff --git a/Makefile b/Makefile
index 1e66c3a..8102da7 100644
--- a/Makefile
+++ b/Makefile
@@ -4,9 +4,7 @@
 APP_ENGINE_PATH=../google_appengine
 
 test:
-	./runtests.sh python2.6
-	./runtests.sh python2.7
-
+	tox
 
 .PHONY: coverage
 coverage: