DiffToTarget needs to support non-whole number difficulties

[jolan]

We didn't think non-whole number difficulties were very common except when using underpowered GPUs but the traces by @unintuitive in #135 make it clear that some pools use them more frequently.

Maybe just rounding up or down would be an improvement.

[jolan]

I don't think the pools were doing strict difficulty checking before because the "round the difficulty down" hack we went with is causing a lot more rejected shares these days.

Since this is supposed to be the reference miner and all, a proper solution should really be implemented.