add some fields to API for displaying more info to miners

this patch adds some fields to local_stats and global_stats web API to
display more detailed infos to miners, especially on p2pool nodes with
more than one miner address attached.

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
                    pairs, which lists the actual difficulty of the latest
                    share dished out to each active miner