Add LogrusLogger interface for Entry and Logger

This make it possible for client code to accept either Logger or Entry.
For example, utility function may accept logger object to inform fatal
errors and it is job of the calling code to provide either generic
top-level logger, or request-bound Entry created using .WithFields.

(fixes #308)

logrus_test.go

@@ -314,3 +314,20 @@ func TestLoggingRace(t *testing.T) {
 	}
 	wg.Wait()
 }
+
+// Compile test
+func TestLogrusInterface(t *testing.T) {
+	var buffer bytes.Buffer
+	fn := func(l LogrusLogger) {
+		b := l.WithField("key", "value")
+		b.Debug("Test")
+	}
+	// test logger
+	logger := New()
+	logger.Out = &buffer
+	fn(logger)
+
+	// test Entry
+	e := logger.WithField("another", "value")
+	fn(e)
+}