Adding per container network stats

1. Added a basic version of network stats inside network package.
2. Introducing a new checkpoint file 'network.stats' which will contain the network runtime information (veth interface names for now).
3. Adding network stats to 'nsinit stats'.
4. Added a libcontainer API to get both network and cgroup stats

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
nit: can we do (for readability):

return NetworkStats{
  Stat1...,
  Stat2...,
  Stat3...,
}, nil