some new features

Fixed issue #107
Implementation issue #98 (add Php::eval)
Add isList and isMap methods for Php::Value
Add isRef and refequals methods for Php::Value
Add ref, keys, properties, hash methods for Php::Value
Add className, id methods for Php::Value
Add is and isSubclassOf methods for Php::Value.
Add Php::is_a, Php::is_subclass_of, Php::class_exists native implementation.
Fixed some other bugs.