Changing work computing function

Issue description at http://www.reddit.com/r/myriadcoin/comments/2oj7y5/lowhashrate_51_attack_on_myriad_without_timewarp/

I originally developed this approach for DigiByte, but never bothered to push.  The idea is to compare work done by each of the 5 algorithms individually, so an attacker has to 51% at least 3 algorithms in order to 51% the network (which is a less than ideal condition, but still seems to be an improvement over the current state).