Working with storage drivers

This PR contains changes need to be made in the ``info`` package:

- ``StorageDriver`` should be able to tell the percentages of CPU/Memory usages and do some sampling works for us.
- ``Percentile`` struct in ``info`` now is visible to the outside world.
- ``Samples`` is not a part of ``ContainerStatsPercentiles`` now.
- Corresponding changes in ``container`` package.

``manager`` package is not affected in this PR. But it will be changed in the following PRs.
WDYT of naming these Get*()? It seems to matchup with AddStats(), but also seems less Go-like. What do you think?