Skip to track updates

this is an option that could be useful to inject an update time for
testing purpose
Track change of fields

with the option `fieldsToTrack` you could track changes to some fields,
for each field a record in `__updates` field is recorded, something like
```json
{ "__updates": [
  { "field": "status",
    "changedTo": "started",
    "at": "Fri Jan 17 2014 15:59:32 GMT+0100"
  }
}
```