Reviewed by Maciej.

    Reviewed by Maciej.

        - Moved the arguments passed to JSClassCreate into a single structure,
        called JSClassDefinition. This will enable easier structure
        migration/versioning in the future, if necessary.

        - Added support for class names.

        - kJSClassDefinitionNull replaces kJSObjectCallbacksNone.

        - JSClass is becoming a fairly complex struct, so I migrated all of its
        implementation other than reference counting to the sruct.

        - Also moved JSClass* functions in the API to JSObjectRef.cpp, since they're
        declared in JSObjectRef.h

        - Also added some more informative explanation to the class structure doc.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@15462 268f45cc-cd09-0410-ab3c-d52691b4dbfc