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.
In general, it's fine. The reason I avoid to use this approach is that ``string(data)`` will allocate a new memory space and copy the content of ``data`` to the new space. Because string is immutable and []byte is mutable, so any conversion between these two types will cause a memory allocation and a copy, which have some performance issues if the data is large.

But I could change it if you think it is a better way to go.