Add fields to local_stats and global_stats web API

https://github.com/donSchoe/p2pool-vtc/pull/7/files
This patch adds fields to local_stats and global_stats web API.

global_stats.network_hashrate: total hashrate of coin network
global_stats.network_block_difficulty: latest block diff in coin network

local_stats.miner_last_difficulties: array of address and share
difficulty, which lists the actual difficulty of the latest share dished
out to each active miner