Implement the splitting of votes

implements #80 (closed)

This allows bakers to cast split votes.

Each ballot contains three positive integers (yays_per_roll, nays_per_roll, passes_per_roll), which indicate the votes for yay, nay and pass per roll, respectively, and such that yays_per_roll + nays_per_roll + passes_per_roll = votes_per_roll where votes_per_roll is a fixed constant.

For each fraction xxx_per_roll, the number of votes that a delegate allocates is xxx_per_roll multiplied by the number of rolls owned by the delegate.