Property name and database column name mapping support request.

Request to support model property name to database column name mapping. Like this:

``` js
var User = orm.define('User', {
    userAccessToken: { type: 'string', column: 'token' }
    lastLogin: String
}, {
...
});
```

The `userAccessToken` map to database column `token`, `lastLogin` auto(or options enable this) map to database column `last_login`.
