Measure memory usages of cgroups

This adds collector for memory of system and its cgroups ( i.e. lxc containers etc ...).

It depends on the existence of memory.stat file for cgroups which can be found at:
```
/sys/fs/cgroup/memory/memory.stat
/sys/fs/cgroup/memory/lxc/memory.stat
/sys/fs/cgroup/memory/lxc/container_name/memory.stat
```
Currently this only measures `cache`, `rss` and `swap`