Removing references from parents

```javascript
var Foo = Backbone.AssociatedModel.extend({});

var Bar = Backbone.AssociatedModel.extend({
       relations: [{
       type: Backbone.One,
            key: 'rel',
            relatedModel: Foo
       }]
});
```
Create some instances:
```javascript
var foo = new Foo;

var bar = new Bar({rel: foo1});
```
Now foo.parents contains one element (bar).
Next, destroy bar.
```javascript
bar.destroy()
// foo.parents.length => 1
```
As you can see, after the removal of foo, parents.length not changed. This is normal, since foo can be used later.
```javascript
bar.save({rel: foo})
```
But there are cases when bar is no longer used and bar must be removed from foo.parents.

This PR helps to solve this problem.
```javascript
bar.destroy({remove_references: true}) // default true
// foo.parents.length => 0
```