Automatic merge from submit-queue (batch tested with PRs 50832, 51119, 51636, 48921, 51712)

Alpha list paging implementation

Design in kubernetes/community#896

Support `?limit=NUMBER`, `?continue=CONTINUATIONTOKEN`, and a `continue` field
on ListMeta and pass through to etcd. Perform minor validation as an example.

```
# first out of three
$ curl http://127.0.0.1:8080/api/v1/namespaces?limit=1
{
  "kind": "NamespaceList",
  "apiVersion": "v1",
  "metadata": {
    "selfLink": "/api/v1/namespaces",
    "resourceVersion": "146",
    "next": "ZGVmYXVsdA"
  },
  "items": [
    {
      "metadata": {
        "name": "default",
        "selfLink": "/api/v1/namespaces/default",
        "uid": "f95e1390-6852-11e7-ab03-7831c1b76042",
        "resourceVersion": "4",
        "creationTimestamp": "2017-07-14T05:12:03Z"
      },
      "spec": {
        "finalizers": [
          "kubernetes"
        ]
      },
      "status": {
        "phase": "Active"
      }
    }
  ]
}
...
# last
$ curl "http://127.0.0.1:8080/api/v1/namespaces?limit=1&continue=a3ViZS1wdWJsaWM"
{
  "kind": "NamespaceList",
  "apiVersion": "v1",
  "metadata": {
    "selfLink": "/api/v1/namespaces",
    "resourceVersion": "145"
  },
  "items": [
    {
      "metadata": {
        "name": "kube-system",
        "selfLink": "/api/v1/namespaces/kube-system",
        "uid": "f95e9484-6852-11e7-ab03-7831c1b76042",
        "resourceVersion": "5",
        "creationTimestamp": "2017-07-14T05:12:03Z"
      },
      "spec": {
        "finalizers": [
          "kubernetes"
        ]
      },
      "status": {
        "phase": "Active"
      }
    }
  ]
}
```