Fall back to image-specified hostname if user doesn't
provide one, instead of only using image-specified
hostname if the user *does* try to set one.
(ditto for username)

Closes #694.

Fixed checksum computing. Ensure checksum is computed when image metadata is loaded from disk. Fixed docker push workflow. Moved hash computing to utils

* This fixes an index out of range crash if cgroup memory is not
  enabled.

Specifically, Ubuntu Precise's cgroup-lite script uses mount -n
to mount the cgroup filesystems so they don't appear in mtab, so
detection always fails unless the admin updates mtab with /proc/mounts.

/proc/mounts is valid on just about every Linux machine in existence and
as a bonus is much easier to parse.

I also removed the regex in favor of a more accurate parser that should
also support monolitic cgroup mounts (e.g. mount -t cgroup none /cgroup).

On Gentoo, the memory cgroup is mounted at /sys/fs/cgroup/memory, but the mount line looks like the following:
memory on /sys/fs/cgroup/memory type cgroup (rw,nosuid,nodev,noexec,relatime,memory)

(note that the first word on the line is "memory", not "cgroup", but the other essentials are there, namely the type of cgroup and the memory mount option)

Update utils.go to not enforce extra constraints on the kernel "flavor" (such as being integral or even comparable one to another)

This is especially to fix the current docker on kernels such as gentoo-sources, where the "flavor" is the string "gentoo", and that obviously fails to be converted to an integer.

Not only is this a more common idiom, it'll make finding bugs easier,
and it'll make porting to Go 1.1 easier.

Go 1.1 will not require the final return or panic because it has a
notion of terminating statements.

Show shorthand image IDs for convenience. Shorthand IDs (or any non-conflicting prefix) can be used to lookup images

Show shorthand container IDs for convenience. Shorthand IDs (or any non-conflicting prefix) can be used to lookup containers

The race condition cen be detected by running existing bufReader tests
with Go's race detector.