Allow mocking for chains

``` php
public function doSomething(MyClass $obj)
{
return $obj->getSomething()->thenSomethingElse();
}
```

``` php
public function testChain()
{
$mock = $this->mock('MyClass')
->stub(['getSomething->thenSomethingElse' => 'foobar'])
->done();
}
```