Creating resources

WIP for creating resources using POST. For more information see the [Updating](http://jsonapi.org/format/) on jsonapi.org.

Todo:

- [x] Add an integration spec
- [x] Update the HTTPClient to support POST requests
- [x] Update the router to support create actions
- [ ] Update the mapper to support serializing objects to `NSDictionary`
- [ ] Update the serializer to support serializing _to_ JSON
- [ ] Tie it all together in the APIClient