Extend existing "meta" solution for JSON REST responses

This change allows custom meta properties returned in JSON responses to be passed to the store by extending the "meta" solution provided in https://github.com/emberjs/data/commit/1787bff.

Meta properties are required to exist in the serializer configuration in order for them to be passed to the store.

    serializer.configure({
        meta: 'meta',
        since: 'since',
        pagination: 'pages'
    });

    payload = {
        meta: {
            since: '123',
            pagination: { total_pages: 2 }
        },
        { results: [...] }
    };

Configured properties will be added to the store `typeMap.metadata` for the appropriate type, under the keys defined in the serializer configuration, eg. `store.typeMapFor(type).metadata.pagination` would be set to the content of the `pages` property of the JSON response.

This affords 1 set of meta properties to be set (and updated) per model type in the store. The meta properties can be accessed in the controller via `this.get('content.store.typeMaps.metadata')`