Fix compile errors with recent mingw.

Constify arguments to copy constuctors. Without this gcc fail to compile if classes used inside containers.
Error message is rather cryptic: 
```
error: 'constexpr std::pair<_T1, _T2>::pair(const std::pair<_T1, _T2>&) [with _T1 = const int; _T2 = cocos2d::experimental::AudioPlayer]' declared to take const reference, but implicit declaration would take non-const
```

I test this:

1. With mingw by successfully create package from MSYS package source [PKGBUILD](https://github.com/Alexpux/MINGW-packages/blob/master/mingw-w64-cocos2dx-git/PKGBUILD)
1. With recent Microsoft Visual Studio Community 2013 (build success, test applications runnable)