add rkt parsing and tarables

[tschuy]

This is by no means done yet.

This adds the github.com/coreos/mayday/mayday/rkt package that is to be used as such:

pods = rkt.GetPods() // returns a list of Pods
for _, p := range pods {
    tbs := p.GetTarables() // returns a []Tarable
    for _, t = range tbs {
        tarables = append(tarables, t)
    }
}

GetTarables() returns three Tarables:

• PodTarable containing the file /containers/{uuid}/pod
• AppTarable containing the file /containers/{uuid}/apps
• LogTarable containing the file /containers/{uuid}/{uuid}.log

So far, if you manually initialize a Pod object with App objects (or use ProcessRktOutput(str) to parse them into existence), you can call GetTarables() to get the Pod and App tarables.

I'm not very happy with this api, but it's at the same time the best I can think of.

Thoughts?