Include header files for upgrade program from 0.12.0 to 2.0-rc2-x-2.1

For version cocos2d-2.0-rc2-x-2.1, the header files are reallocated into separated folders which is a good idea.

However, we check out the include file “cocos2d.h”, it is noted that the include statement does not include the folder path. We need to add include path in project solution to find those files.

It is supposed that the project have to explicitly mention correct links to avoid confusion or ambiguous setting. Thanks