Various miner incentives

This incorporates changes suggested by #4.
For simplicity, we assume that the only value in the block reward is the mining fees.
For simplicity, we assume that there are no limits in block size, and that a reorging miner can take all fees in both blocks.

E(mining block) = sum of all fees on current block * p(mining block t)
E(mining previous block to realloc fees) = (sum of all fees on current block + prev block) * p(mining block t-1) * p(mining block t)
E(mining prev) > E(mining current) when:  (prev fees + current fees) * p(mining) * p(mining) > current fees * p(mining).
p(mining) := 0.5: E(mining prev) > E(mining current) when prev fees > current fees
p(mining) := 0.4: E(mining prev) > E(mining current) when prev fees > current fees * 1.5