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%
Nit, this should never error, but... I think the headers have already been written at this point. It might be better to call json.Marshal before setting the headers.