Tribute contract implementation

[dusan-maksimovic]

Closes #7

The three main messages are kept pretty much the same way they were, except for the part that they don't consider only the winner proposal, but the top N proposals. Round ID has been removed from AddTribute message, since it is only allowed to add tributes for the ongoing round. The code that calculates the amount of tribute that goes to each voter is adjusted to calculate percentage correctly.

[p-offtermatt]

Just out of an abundance of caution, we should make sure that if a proposal receives no support at all, we allow refunding its tribute.

[dusan-maksimovic]

Just out of an abundance of caution, we should make sure that if a proposal receives no support at all, we allow refunding its tribute.

We are good here with our current implementation, since the TopNProposals query uses PROPS_BY_SCORE to fetch the proposals, and if nobody voted for proposal it won't even be in this map, since it is only inserted during vote() message execution. So, this means that get_top_n_proposal() will return None if someone tries to refund the tribute for the proposal with 0 votes.

[p-offtermatt]

Nice, thanks! LGTM