+ [feature] supports the enum/in can use object to transform value

these PR MUST BE applied first:

* https://github.com/balderdashy/anchor/pull/67
* https://github.com/chriso/validator.js/pull/336

see https://github.com/balderdashy/waterline/issues/52


```json
attributes: {
  state: {
    type: 'int',
    enum: {'pending':2, 'approved':3, 'denied':4]
  }
}
```