fix idempotecy.

M
add stdlib to fixtures

this is required for successfull rake spec tests.
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
rspec-system test to cover the new ipv6 feature
add option to manage install of radvd package