Add basic support for IP addresses

[bkircher]

Add command to list, remove, create IP -4 and -6 addresses. Add a way to assign them to servers.

Maybe something like

gscloud ip ls # lists all
gscloud ip ls -4 # lists only IPv4
gscloud ip ls -6 # lists only IPv6
gscloud ip rm <id> # delete by ID or
gscloud ip rm <address> # delete by address
gscloud ip create --reverse-dns=example.com --assign-to=<server id> # will pull a IPv4 from pool
gscloud ip create -6 --reverse-dns=example.com --assign-to=<server id> # same with IPv6
# supporting additional flags like
 --name=Example
 --failover=yes

[bkircher]

Done with 3409fd4