Adds a property to check whether the object is a native associated object.
All classes exposed from Native to JS should contains a property named `__nativeObj` of which value is `true`.
Adds a new line in layout_head.c.
Reverts to use static for function and property definition. I don’t know whether removing it will cause problems. :(
Merge pull request #77 from dumganhar/native-obj-check

Adds a property to check whether the object is a native associated object.