readded minimum difficulty change
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
add new web frontend in /web-static.modern

Provides a new look and some more infos for miners.
For use, please rename the /web-static.modern dir to /web-static

See /web-static.modern/README.md for more info.

See https://github.com/johndoe75/p2pool-node-status for the original version
of this script.
Sets a minimum pseudoshare target to avoid diff 0 HW errors, and sets the targets based on address speed not local node total speed
readded roy7 and mmouse's patches from don's fork