Exit with code 0 or 1

Using the number of failed tests as the exit code results in exit code
0 if a multiple of 256 tests fail.  Fixes github PR#10, but in a
slightly different way.

runtests.py

@@ -9,4 +9,8 @@
 else:
     num_failures, num_tests = doctest.testfile('docs/quickstart.txt')
 
-sys.exit(num_failures)
+if num_failures > 0:
+    print("%d failures out of %d tests" % (num_failures, num_tests))
+    sys.exit(1)
+
+sys.exit(0)