schema, discovery: add new functions and convert discovery.App.Labels to use types.Labels.

Hi,

probably these changes won't have big purpose per se, but they're the appc/spec parts that I'm needing for work on #16. I'm going to open a big RFC PR on coreos/rocket for the aci renderer part.

In the discovery part I moved App.Labels from a map to a types.Labels, this will make the discovery more consistent with the spec and avoids conversions from map to types.Labels when, for example, I'm creating a discovery.App using NewAppFromString and I want to pass this App.Labels to store.GetACI.