Update HTTPClient to use C++11 callbacks

[NEW]: update HTTPClient to use C++11 callbacks
[NEW]: updated tests to use the new API
[DEP]: deprecated old target/selector API

I tried to follow the conventions found in `CCMenuItem` as much as possible.

**Note**: This fixes [issue #4702 on the issue tracker](http://www.cocos2d-x.org/issues/4702).
Thanks,
Maz