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 :)

The setup process is run in the root userns.  The init is started (cloning into userns) and then waits for setup to run and set things up. Setup joins all the namespaces of init except userns. Thanks for taking a look. Feedback and suggestions appreciated.