Automatically create PTR record

This patch adds automatic creation of PTR records for docker containers, so container IP addresses can be resolved into hostnames as well:
```
root@c94e086699b8:/# host 172.17.0.5
5.0.17.172.in-addr.arpa domain name pointer skydock.machines.skydns.local.
root@c94e086699b8:/#
```