change paritipantsIndex from hashmap to array

[makoto]

I changed participantsIndex from hashmap to array. This has a slight penalty on gas cost (contract create 1686592 -> 1764065, and register 119996 -> 120265), but it enables me to get the whole array using getParticipants function. I believe that this was impossible with hashmap. This change should retain the interface exactly as is.

[makoto]

One of the problems is that this actually breaks compatibility with old contract as the old contract does not have getParticipants. if I change dapp js to use getParticipants, it won't work against old contract anymore.