Merge pull request #1 from lud4ik/master

Fix typo in docs
Implement setting gid for namespaces
Add setuid/setgid/setgroups in container
First part of docker download script
Add `_extract_json` command
Remove '\t' after last column
Implement empty json field
Fully implement docker pull
Implement preliminary Dockerfile support
Documented docker builder
Implement container version tracking for docker builder
Preliminary subuid and subgid support
Fix bug in subuid parsing, add `vagga _userns` command

The `vagga _userns` command is very similar to lxc-usernsexec, but with
vagga flavour. Basically it should replace lxc-usernsexec in builders.
Always build under user namespace (not all backends tested)
Fix debian-simple builder
Fix other image-based builders
Do not double-unshare user namespace
Add first version of _clean command
Using user namespaces for removing old container build artifacts
Add big nice label on process shutdown