Added LogRoller parser and entity.

The errors and logs can now have log rolling if provided by the user.
The current customisable parameter of it are:
The maximal size of the file before rolling.
The maximal age/time of the file before rolling.
The number of backups to keep.

middleware/roller.go

@@ -0,0 +1,25 @@
+package middleware
+
+import (
+	"io"
+
+	"gopkg.in/natefinch/lumberjack.v2"
+)
+
+type LogRoller struct {
+	Filename   string
+	MaxSize    int
+	MaxAge     int
+	MaxBackups int
+	LocalTime  bool
+}
+
+func (l LogRoller) GetLogWriter() io.Writer {
+	return &lumberjack.Logger{
+		Filename:   l.Filename,
+		MaxSize:    l.MaxSize,
+		MaxAge:     l.MaxAge,
+		MaxBackups: l.MaxBackups,
+		LocalTime:  l.LocalTime,
+	}
+}