Allow mocking for chains

[elliotchance]

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

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