moved the DS.RecordArray creation of Store#all(type) into a separate function

Previously Store#all had the creation of the returned DS.RecordArray hard coded into the function:
```
var array = DS.RecordArray.create({ type: type, content: Ember.A([]), store: this, isLoaded: true });
this.registerRecordArray(array, type);
```
This pull request gives the user an opportunity to supply a derived type:
```
    //Store#all
    var array = this.newRecordArray(type);

    this.registerRecordArray(array, type);
```
```
  newRecordArray: function(type){
    return DS.RecordArray.create({ type: type, content: Ember.A([]), store: this, isLoaded: true });
  },
```
For example,  I might want to supply a PagedRecordArray that provides paging functionality.