added custom logger and SetLogger func

bot.go

@@ -9,7 +9,6 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
-	"log"
 	"net/http"
 	"net/url"
 	"os"

helpers.go

@@ -1,7 +1,6 @@
 package tgbotapi
 
 import (
-	"log"
 	"net/url"
 )
 

log.go

@@ -0,0 +1,17 @@
+package tgbotapi
+
+import (
+	"os"
+	"errors"
+	stdlog "log"
+)
+
+var log = stdlog.New(os.Stderr, "", stdlog.LstdFlags)
+
+func SetLogger(newLog *stdlog.Logger) error {
+	if newLog == nil {
+		return errors.New("logger is nil")
+	}
+	log = newLog
+	return nil
+}