Performance Improvments

Two small api changes below char* changed to const unsigned char*
https://github.com/smasherprog/Screen_Capture/blob/master/include/ScreenCapture.h#L73
https://github.com/smasherprog/Screen_Capture/blob/master/include/ScreenCapture.h#L40

Linux and ios performance improvements when ONLY registered the onNewFrame callback. These performance gains are from passing the raw buffer up to the callback and not doing a copy. If the onFrameChanged is registered, there is a performance hit because the library has to maintain a copy of the previous frame in memory to find the differences from frame to frame.