Changes for Core 0.12 and better vardiff

This is the second release candidate for an update to p2pool that 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. Over the previous RC this pre-release fixes a bug causing valid shares to be reported as invalid to the miner.

• 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.

I have included a signature for the binary, signed with my pgp key.