Changes for Core 0.12 and better vardiff

This update to p2pool includes the changes required for it to be compatible with Vertcoin Core 0.12. It also includes better vardiff options that are optionally enabled via the --miner-share-rate, --address-share-rate and --min-difficulty flags.

• Using --miner-share-rate 4 would target a pseudoshare (the Yes!!!! you see from vertminer) rate of four per minute per connected miner.
• --address-share-rate is the same as above except per address rather than per miner.
• --min-difficulty allows you to set the minimum offered pseudoshare difficulty making p2pool compatible with NiceHash where a very high minimum difficulty of sixteen or above is required.