middleware/log: allows logging based on response classes (#325)

Add the ability to add a class of responses to be logged; success,
denial or error. The default is to log everything (all).

Fixes #258

middleware/cache/cache_test.go

@@ -94,7 +94,7 @@ func TestCache(t *testing.T) {
 		m = cacheMsg(m, tc)
 		do := tc.in.Do
 
-		mt, _ := response.Classify(m)
+		mt, _ := response.Typify(m)
 		k := key(m, mt, do)
 		crr.set(m, k, mt, c.pttl)