Add an bility to define inheritance for interfaces

This small patch enables interface inheritance to complete basic OO model of PHP.

PHP side:
```php
interface A {}
interface B extends A {}
class C implements B {}
```

C++ side
```c++
Php::Interface a("A");
Php::Interface b("B");
Php::Class<C> c("C");

b.extends(a);
c.implements(b);
```