Measure cpu's total user and system time of system and cgroups

This adds collector for cpuacct ( which collects utime and stime ) of system and its cgroups ( i.e. lxc containers etc ...).

It depends on the existence of `cpuacct.stat` file for cgroups which can be found at:
- `/sys/fs/cgroup/cpuacct/cpuacct.stat`
- `/sys/fs/cgroup/cpuacct/lxc/cpuacct.stat`
- `/sys/fs/cgroup/cpuacct/lxc/container_name/cpuacct.stat`

Contents of `cpuacct.stat` file will be:
```
user 1234 # total number of user time for this cgroup
system 3456 # total number of system time for this cgroup
```