Add server based pagination support

ember-data-django-rest-adapter is now able to handle paginated results from the server. Paginated results are automatically discovered by looking for a `results` array in the response from django-rest-framework. A new `pagination` object is added to the metadata with these properties:
- `count`: total number of objects
- `current`: current page number
- `previous`: previous page number, or `false`
- `next`: next page number, or `false`

fixes #24