provide capability for process to be aware of PID of daemon


#### To make your daemon pid-aware

* *PersistPID* : to persist PID of daemon process in a (default or path passed as param) pidfile 
* *KillPID* : to kill PID from process' pidfile
* *StatusPID* : to check status of PID from process' pidfile

```go
package main

import (                                                                        
  "flag"                                                                        
  "fmt"                                          
                                                                                
  "github.com/ChaosCloud/godaemon"                                              
)                                                                               
                                                                                
var (                                                                           
  axn = flag.String("axn", "status", "status|start|stop")                       
)                                                                               
                                                                                
func main() {                                                                   
  godaemon.MakeDaemon(&godaemon.DaemonAttr{})                                   
  flag.Parse()

  switch *axn {                                                                 
  case "stop":                                                                  
    fmt.Println("Stopping...")                                                  
    if godaemon.KillPID("") {                                                   
      fmt.Println("Status: Stopped.")                                           
    } else {                                                                    
      fmt.Printf("Failed to stop. Status: ")                                    
      godaemon.StatusPID("")                                                    
    }                                                                           
  case "status":                                                                
    fmt.Printf("Status: ")                                                      
    godaemon.StatusPID("")                                                      
  default:                                                                      
    if godaemon.PersistPID("") {                                                
      // call whatever process is actually gotta run as daemon                                                         
    }                                                                           
  }                                                                             
} 

```