Use bits operators to merge all "accepts votes" in a single storage
entry, and all "reject votes" in another storage entry.

We now support up to 255 participants.
No matter how many participants there are a transaction always use
only 5 storage entries (versus 3 + nbParticipants previously)
Refactor contract storage

No more master `for` loop to find the participant index, instead we
associate the address and the index in the contract storage.
The GAS price is now the same for every participant.