add last version
testing, issue #1104: Make the test use static flags
change tag -> repo name in build usage
Added version checker interface
inserted setUserAgent in each HTTP request
added APIVersion when call NewRegistry
Insert version checkers when call NewRegistry()
added client's kernel version
Removed an unnecessary error check.
Removed an unnecessary nil assignment
format in the user agent header should follow RFC 2616
reduce the number of string copy operations.
use http://get.docker.io/latest
docker.upstart: avoid spawning a `sh` process

start script / end script create an intermediate sh process.
docker.upstart: use the same start/stop events as sshd

Is probably more solid
+ Runtime: inject dockerinit at /.dockerinit instead of overwriting /sbin/init. This makes it possible to run /sbin/init inside a container.
documentation.
Merge branch 'master' into add_last_version-feature
handle -dev
add ps args to docker top