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%
If encoder wrote any data to w before failing, I think the headers will have already been written. I could be wrong, though...