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.
We don't have to. But initializing with a size would pre-allocate the memory space and makes the following appending operations faster. Here is a result of a micro-benchmark I just wrote:

```
$ go test -bench=.
PASS
BenchmarkMapInitWithSize	  500000	      6505 ns/op
BenchmarkMapInitWithoutSize	  200000	     11192 ns/op
ok  	github.com/google/cadvisor/storage/memory	5.694s
```