Support for setting up access points using hostapd and dnsmasq

New wrappers for manipulating hostapd and dnsmasq configuration
files and an AccessPoint class that combines hostapd, dnsmasq
and scheme in order to set up an access point with dhcp, dns
forwarding, local domain resolution and optionally also NATing
to an existing interface.

Also adds support for static interfaces.

New dependency on the netaddr module had to be added in order
to make the ip network handling for the access point mode less
cumbersome.

Scheme now also support overwriting of existing configurations
and more than one entry per configuration key in extended options
(necessary for adding iptables/sysctl calls for post-up/pre-down
events in /etc/network/interfaces).
The problem with that though is that the chipset specific hostapds mostly don't support other chipsets at all. E.g. with one compiled for the `rtl8192cu` (Edimax EW-7811Un, also quite common) and necessary `rtl871xdrv` driver you won't be able to fire up an AP when using something with a `rt2800usb` chipset (e.g. BIG 687 or WiPi). It's a mess.

I've been meaning to get around this with a detection script for OctoPi that not only preconfigures the driver to use but also the hostapd binary. But I'm up to the neck in other stuff atm, so any help with that is welcome ;)