Custom column property name

Added ability to create a definition using custom property names for columns. Example:

```javascript
var user = sql.define({
  name: 'user',
  columns: [{
      name: 'id'
    }, {
      name: 'state_or_province',
      property: 'state'
    }
  ]
});

console.log(user.select().where(user.state.equals('WA')).toQuery().text);
// "SELECT "user".* FROM "user" WHERE ("user"."state_or_province" = $1)"
```