Expose idType constructor on service

This is so that calling code can just lookup the service rather that both the service *and* the persistence layer.

This:

```js
var articleService = serviceLocator.articleService

new articleService.idType('515428002e906a2c84000001')
```

instead of this:

```js
var ObjectId = serviceLocator.persistence('article')
  , articleService = serviceLocator.articleService

new ObjectId('515428002e906a2c84000001')
```