Integrated instance addresses and networks

Moved several items from the instance package
to the network/ package, renamed some
of them, and propagated the changes across:
 - instance.Address -> network.Address
 - instance.AddressType -> network.AddressType
 - instance.HostName -> network.HostName
 - instance.Ipv4Address -> network.IPv4Address
 - instance.Ipv6Address -> network.IPv6Address
 - instance.NetworkScope -> network.Scope
 - instance.Address.NetworkScope -> n.Address.Scope
 - instance.Network* -> network.Scope*
 - instance.NewAddress(es) -> network.NewAddress(es)
 - instance.HostPort -> network.HostPort
 - instance.Port -> network.Port
 - instance.Select*() -> network.Select*()
 - instance.SortPorts -> network.SortPorts

(I might have missed some, but you get the idea).

Also, some refactoring was done to split addresses and
ports implementation in separate files and test suites.

No other logic changes introduced - the rest is just
mechanically applying the changes above.

Moved environs/network to network/ and updated imports,
as suggested on the review.
Merge pull request #49 from dimitern/510-instance-addresses-and-networks-integration

Moved several items from the instance package
to the network/ package (used to be environs/network/),
renamed some of them, and propagated the changes across:
 - instance.Address -> network.Address
 - instance.AddressType -> network.AddressType
 - instance.HostName -> network.HostName
 - instance.Ipv4Address -> network.IPv4Address
 - instance.Ipv6Address -> network.IPv6Address
 - instance.NetworkScope -> network.Scope
 - instance.Address.NetworkScope -> n.Address.Scope
 - instance.Network* -> network.Scope*
 - instance.NewAddress(es) -> network.NewAddress(es)
 - instance.HostPort -> network.HostPort
 - instance.Port -> network.Port
 - instance.Select*() -> network.Select*()
 - instance.SortPorts -> network.SortPorts

(I might have missed some, but you get the idea).

Also, some refactoring was done to split addresses and
ports implementation in separate files and test suites.

No other logic changes introduced - the rest is just
mechanically applying the changes above.