Use user namespaces

On new enough kernels, sandbox-supervisor now works without being
setuid root.  I know of two regressions:

 * --proc is broken when user namespaces are used.  Fixing this is possible
   but tricky.

 * Running sandbox-supervisor as root with user namespaces fails.  This
   happens because, once root has entered the namespace, it has insufficient
   privilege to map anyone else into the namespace.  This should be
   straightforward to fix.
sandbox-supervisor: Remove non-userns support and all uid and gid code

This now requires CONFIG_USER_NAMESPACE.  It is no longer safe to install
setuid root.  It is probably unwise to run it as root.  The code
simplification should be worth it.
sandstorm-supervisor: Fix --proc
Merge remote-tracking branch 'amluto/userns'

Conflicts:
	src/sandstorm/supervisor-main.c++
supervisor: With --proc, bind-mount procfs recursively

This is useful: it makes /proc more useful inside the container.  It
also works around a kernel bug that was introduced by Linux commit
e51db73532955dc5eaba4235e62b74b460709d5b and fixed in
41301ae78a99ead04ea42672a1ab72c6f44cc81d.
supervisor: Stop relying on /usr existing for the --proc shuffle

We can use "/".  Yes, this is ugly.
Add other core contributors to front page.
Merge branch 'userns' of github.com:amluto/sandstorm into amluto-userns

Conflicts:
	src/sandstorm/supervisor-main.c++
Stop setting suid-root on sandstorm-supervisor.
Tweak style from PR #12.
Make /dev/random synonymous with /dev/urandom in the sandbox. In practice there is no reason to prefer /dev/random anyway, and exposing it makes it trivial to DoS any other process on the system that itself uses /dev/random.
Merge branch 'amluto-userns'
Merge branch 'master' of github.com:sandstorm-io/sandstorm