Native Client port

Hi guys,

This is an initial port of cocos2dx to Native Client (NaCl).

I've tried to follow your coding standards and put you license header at the top of all
new source files.

You can see the port in action (if you are using chrome) here:

https://chrome.google.com/webstore/detail/cocos2dx-test-app/hbnepalailfleigebaanjnoiapedgcpn