Exist系列方法

判断某个记录是否存在可以使用Exist, 相比Get，Exist性能更好。

has, err := testEngine.Exist(new(RecordExist))
// SELECT * FROM record_exist LIMIT 1

has, err = testEngine.Exist(&RecordExist{
        Name: "test1",
    })
// SELECT * FROM record_exist WHERE name = ? LIMIT 1

has, err = testEngine.Where("name = ?", "test1").Exist(&RecordExist{})
// SELECT * FROM record_exist WHERE name = ? LIMIT 1

has, err = testEngine.SQL("select * from record_exist where name = ?", "test1").Exist()
// select * from record_exist where name = ?

has, err = testEngine.Table("record_exist").Exist()
// SELECT * FROM record_exist LIMIT 1

has, err = testEngine.Table("record_exist").Where("name = ?", "test1").Exist()
// SELECT * FROM record_exist WHERE name = ? LIMIT 1