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.
If you are going to expose this in the public API, please clearly document the meaning of the measurements.
 - What is the denominator of the percentage?  Of the container's reservation? Its limit? 
 - Over what time period is the information averaged or maxed?  When was this measurement taken?