added containment relationship for networks

The current version of the ipaddress library implements containment relationship in a way that a network is never contained in another network :

    >>> from ipaddress import IPv4Network,IPv4Address
    >>> IPv4Network(u'192.168.22.0/24') in IPv4Network(u'192.168.0.0/16')
    False

I think it would be better to define the containment relationship between networks as such :
- if network A contains all the ip addresses of network B, then B in A is True
- by extension of this rule, A in A is True

It is useful to quickly determine if a network is a subnet of another