Add support of nested model / namespaces

[moaminsharifi]

How to reproduce issue?

We have a app/Model/ dir like that:

.
├── Administrator
│   └── Accounting
│       └── UserSub.php
├── Administrator.php
└── User.php

As we excepted to have

• UserSub
• Administrator
• User in drop down options but there is n't any UserSub:

It's seems this package not support nested name space.

BTW, I will try to make it and create PR for it.