network -> cocos2d::network

Change namespace of network to cocos2d::network. Can not use namespace alias like this

```cpp
namespace network = cocos2d::network;
```

Because if define a namespace alias like this, there will be compiling error in the codes

```cpp
USING_NS_CC

WebSocket socket = new network::WebSocket(); // Doesn't know which `network` to use.
```cpp