add support for IPv6 in libvirt::network

as of yet this only supports IPv6 address, not routes. But then again
we don't support IPv4 routes yet either ;)

We also add spec tests as per class and README documentation.
adapt spec_helper_system to work w/o version

make the code just a *little* prettier.
rspec-system test to cover the new ipv6 feature
add option to manage install of radvd package
Merge pull request #14 from igalic/ipv6

Implement IPv6 for libvirt::network