Added Class<T>::Class<T>& implements(ExtInterface *) :arrow_down: explanation inside

This method allows extend the library php-cpp to external php interfaces.
For example, let InterfaceCountable inherited from ExtInterface and provides interface Countable from php SPL.
InterfaceCountable implemented as a separate library (plug-in).
In this case, that would declare the interface implementation InterfaceCountable we will need to apply
.implements(new InterfaceCountable())

Let library phpsplcpp.so has header file phpsplcpp.h
In our php-extension we write:

class MyTestExt : public Php::Base
{
	...
}

....

extern "C"
{
    ...
        extension.add(
                "MyTestClass",
                Php::Class<MyTestExt>({

                    ...

                }
            ).implementsArrayAccess()
             .implements(new PhpSpl::InterfaceCountable()) // InterfaceCountable defined at phpsplcpp.h
        );
    ...

Of course, we can use this way and our `ExtInterface`
I.e. instead

        extension.add(
                "MyTestClass",
                Php::Class<MyTestExt>({

                    ...

                }
            ).implementsArrayAccess()
        );
we can write
        extension.add(
                "MyTestClass",
                Php::Class<MyTestExt>({

                    ...

                }
            ).implements(new Php::InterfaceArrayAccess())
        );