1. make lua_close work for wxlua bind app, since it will trigger lua's gc work.
When gc work, all object created in lua will be destroyed by gc
so that luabind will be called to delete any wx object created in lua.
before, it will crash, now it will not.
All wx windows object should not be destroyed by lua'gc, but by c++ itself.
top window destroy will destroy all children windows.
Use solution:
class_<class, class*> to register one class whose life owned by c++, not by lua
macro:
BEGIN_BIND_CPP_CLASS(class)

2. provide wxT/_T/_ function to auto convert lua string to wxString object.
so that lua code can use wx._T("this is lua string") to pass any one wx function
which required wxString as argument.

3. fix one bug in wxLuaApp function, after calling lua_close function.