Auto serial id generation support when user post data.

- To create serial id filed while create a table automatically if
  request header has "x-pgrest-create-identity-key: yes".

- curl example:

```
$ curl  -X POST -H "Content-Type: application/json" \
        -H "x-pgrest-create-identity-key: yes"  \
        -d '{"a":1}' http://localhost:3000/collections/table
```