Localforage key fix

This is fix #9.
This should not be BC, as the `localeforageKey` is now added to the context instance. The `model.sync` being a function it is better IMO, to set the `localeforageKey` on the instance itself.

before:  `model.sync.localeforageKey`
after: `model.localeforageKey`

Test added.

This PR is based on my other PR #13.