Research way to use kinto.js without kinto-http.js

Using `kinto.js` without `kinto-http.js` is a totally viable solution. We should figure out a reasonable way to enable that use case for users that want a smaller bundle and are okay with giving up sync abilities.

I think this can be accomplished if we export KintoBase and the user omits adding a `ApiClass` getter. We could probably also handle this in the library by creating our own class like `OfflineKinto` or something.