XML-RPC server: Add IPv6 support

SimpleXMLRPCServer uses AF_INET as default address family.
This means it can only bind to IPv4 addresses.

GlancesXMLRPCServer is a wrapper class that makes it able to
listen on IPv6 connections.
The AF_INET or AF_INET6 address family ìs now determined by
socket.getaddrinfo() automatically.
This way the code is independent from the stack IP version.
Add batteries info thanks to Batinfo lib
Merge branch 'batinfo'
Add batteries information (batteries % capacity is display in the footer/middle)
Add batinfo to setup.py
Fix issue #258
flake8 fixes
Add psutil >= 1.0.0 support

Add support to the new net_io_counters() API.
Of course, still preserved backward compatibility with the now deprecated
network_io_counters().
Correct a bug on Batinfo __init__ function
Add control on io_counters sort | Issue #250
Update README

Remove rst note directive because not supported by GH.
Add batinfo as new optional dependency.
Add FreeBSD installation instructions.
Merge remote-tracking branch 'upstream/master'