Add features from README

json_formatter.go

@@ -0,0 +1,17 @@
+package logrus
+
+import (
+	"encoding/json"
+	"fmt"
+)
+
+type JSONFormatter struct {
+}
+
+func (f *JSONFormatter) Format(entry *Entry) ([]byte, error) {
+	serialized, err := json.Marshal(entry.Data)
+	if err != nil {
+		return nil, fmt.Errorf("Failed to marshal fields to JSON, %v", err)
+	}
+	return append(serialized, '\n'), nil
+}