added processDataForTimestamp to GPUImageRawDataInput

added - (void)processDataForTimestamp:(CMTime)frameTime; so that we can provide a valid timestamp to the target. The motivation of doing this can be found here:
http://stackoverflow.com/questions/22619944/gpuimage-synchronization-of-2-branch-chains/22645701?noredirect=1#comment34620435_22645701