Add and validate namespace to pod, service, replication controller objects

Next iteration in delivering code against the integrated namespaces proposal, the PR adds the following:

1. Add Namespace to Pod, Service, ReplicationController objects
2. Enforces Namespace is set prior to persistence
3. Enforces context.Namespace on create and update operations matches resource.Namespace

Feedback welcome.

I can't do this because our object does not implement all the methods of context yet.

```
_output/go/src/github.com/GoogleCloudPlatform/kubernetes/pkg/api/context.go:46: cannot convert parent (type Context) to type context.Context:
	Context does not implement context.Context (missing Deadline method)
```