responses to review comments

- empty string as marker for failure to discover calling function
 - tighten up logger usage - don't rely on std logger internally

Also fix ordering of expected/got in logrus_test.go to ensure correct
output form test failures.

json_formatter_test.go

@@ -193,6 +193,7 @@ func TestFieldDoesNotClashWithCaller(t *testing.T) {
 func TestFieldClashWithCaller(t *testing.T) {
 	SetReportCaller(true)
 	formatter := &JSONFormatter{}
+	std.ReportCaller = true
 
 	b, err := formatter.Format(WithField("method", "howdy pardner"))
 	if err != nil {