Add a rollingupdate lib and command to kubectl

First take at a rollingupdate lib/command using approach proposed in #1353. Putting it in kubectl to unblock #2575 and #2576. As I understand it, a decision was made to not have rollingupdate be part of kubectl, but the logic is a bit too involved for a bash script in contrib/rctools/, and adding yet another client tool seems like a usability regression. Happy to discuss.

@bgrant0607, @lavalamp 
Use RC updating should be using the normal client behavior (client.Client) instead of a RESTHelper.

Use:

```
client, err := f.ClientBuilder.Client()
checkErr(err)
```

instead here.  I don't think you need a generic client to deal with replication controllers because we don't have any short term plans to do rolling update on anything other than RCs.

With that in mind, a lot of the client code can become much simpler because you can work off of a client rather than a mapping.