support DB.BeginTx of golang 1.8

Signed-off-by: Penghui Liao <liaoishere@gmail.com>

orm/orm_log.go

@@ -15,6 +15,7 @@
 package orm
 
 import (
+	"context"
 	"database/sql"
 	"fmt"
 	"io"
@@ -150,6 +151,13 @@ func (d *dbQueryLog) Begin() (*sql.Tx, error) {
 	return tx, err
 }
 
+func (d *dbQueryLog) BeginTx(ctx context.Context, opts *sql.TxOptions) (*sql.Tx, error) {
+	a := time.Now()
+	tx, err := d.db.(txer).BeginTx(ctx, opts)
+	debugLogQueies(d.alias, "db.BeginTx", "START TRANSACTION", a, err)
+	return tx, err
+}
+
 func (d *dbQueryLog) Commit() error {
 	a := time.Now()
 	err := d.db.(txEnder).Commit()