Add hexadecimal color support

Hello Cocos2D team !

I just add some basic methods to easily use hexadecimal colors in Cocos2D. This is more easy and fast to copy and paste a hexadecimal color value from your favorite image editor than to copy and paste three values for RGB.

As I said, just a really tiny contribution. :-)

Thank you very much for this awesome IPhone framework and have a good day.

Tony