in memory storage

This is an implementation of in memory storage driver. It is basically a rewrite of statssum.go in container package.

Other changes is on the way.
The reason why I did a deep copy here is that I don't want the StorageDriver to take the ownership of the stats. To the caller, StorageDriver is something like a file/database. So ``AddStats`` should follow the semantic of ``Write()`` method. Whatever sends to ``Write()``, the caller should be free to change it after calling the ``Write()`` function and it should not affect the following write operations. If it takes the ownership of stats, it seems like ``Write()`` function takes the ownership of the buffer passed into it.