Adds extractMeta support so that Parse's count parameter is returned as metadata

If you pass `count=1` to Parse's REST API, it will include a total count of records in the response but it's outside the `results` part of the payload so you can't access it using the current ember-parse-adapter. This pull request grabs that count and adds it to the model's metadata.

The following route code is an example where you might be paginating results:

```javascript
model: function(params) {
  var limit = 10;
  var skip = params.page * limit;
  
  return this.store.find("my_parse_model", {limit: limit, skip: skip, count: 1});
}
```

To get the total count from the meta, you would simply add a controller property like this:

```javascript
totalNumberOfRecords: function() {
  return this.store.metadataFor("my_parse_model").count;
}.property("totalNumberOfRecords")
```