Release version 0.6.1

• Fix spelling at MySQL Plugin (by koemu)
  #47
• Change scale KB into Byte about JVM memory usgae (by y-uuki)
  #50
• JVM pidfile option (by y-uuki)
  #51
• Add GC time percentage graph (by y-uuki)
  #52
• Elasticsearch: not exit but warn when some values cannot be fetched (by naokibtn)
  #53
• Fix MySQL int32 overflow (by y-uuki)
  #54