Add Php::is_a implementation.

Add is, isSubClassOf for Php::Value.
Add Php::is_a implementation.
Add Php::is_subclass_of implementation.
Merge branch 'is_a' of https://github.com/andot/PHP-CPP
added Value::subclassOf(), and implemented Php::is_a() and Php::is_subclass_of()
removed includes for fastcall.h
fixed compile issues