Allow a path as the return value of the keyForXXX.

Consider the following JSON: `{ dashboard: { content: { tabs: [] } } }`

```javascript
App.Dashboard.reopen({
    tabs: DS.hasMany(App.Tab)
});
App.DashboardAdapter.map('App.Dashboard', {
    tabs: { embedded: "always" }
});

App.DashboardSerializer = DS.JSONSerializer.extend({
    keyForHasMany: function(type, name) {
        return 'content.'+name;
    }
});
```