support of the timestamp column

- support of the timestamp column

- as an example, it can use in "ad delivery" log table.
```
[
    { "name":"time", "type":"timestamp" },           
    { "name":"showed_at", "type":"timestamp" },      
    { "name":"clicked_at", "type":"timestamp" },     
    { "name":"converted_at", "type":"timestamp" },   
    〜 any other column 〜
]
```