add better parents support + some new methods

- Parent keys are found via `this.parents` array
- Remove `primaryKey`, add `primaryKeys`
- Add #delete
- `errors` is a computed property
- `isPersisted` relies on `created_at`, by default
- `pick` selects specific attributes as a plain object
- `::all` and `::find` accept a `parents` option and add it to the
results
- support for `getBeforeSend`
- support for `forceArray`
v0.2.0