Letting kubelet retrieve container stats from cAdvisor

This PR adds a new API into the Kubelet server which could return statistical information about a container. Such information is retrieved through cAdvisor REST API.

This PR introduced the following third party dependencies:

- [cAdvisor](http://github.com/google/cadvisor) because we need to use some data structure there and its client library.
- [testify](https://github.com/stretchr/testify) for its mock library
- [objx](https://github.com/stretchr/objx) because testify depends on it.

The test coverage now for ``pkg/kubelet`` is improved from 58.1% to 59.9%

The current code is not wired with kubelet now. Will add it in another PR.
I'd skip using the encoder, I think its more complicated than necessary, and possibly will mess up headers too.

How about:
data, err := json.Marshal(stats)
if err != nil {
  ...
}
w.WriteHeader(http.StatusOK)
w.Header().Add("Content-type", "application/json")
fmt.Fprint(w, string(data))