Adds user namespace support to libcontainer

The build will fail as this depends on golang 1.4
I am sending the PR, hoping to get people to test it and give feedback.
There are still log statements for debugging that I will clean up (but keeping them now to make it easier to catch issues).

@crosbymichael @vmarmol @rjnagal PTAL and try it out :)

mknod isn't allowed in a non-root userns