Add new hello-world image

Fixes #102

```console
root@60aa8ed6c0f4:/stackbrew/stackbrew# ./brew-cli --debug -b hello --targets hello-world git://github.com/tianon/stackbrew
2014-07-09 19:58:26,227 DEBUG Cloning repo_url=git://github.com/tianon/stackbrew, ref=hello
2014-07-09 19:58:26,228 DEBUG folder = /tmp/tmpnum1Rv
2014-07-09 19:58:26,228 DEBUG Cloning git://github.com/tianon/stackbrew into /tmp/tmpnum1Rv
2014-07-09 19:58:26,228 DEBUG Executing "git clone git://github.com/tianon/stackbrew ." in /tmp/tmpnum1Rv
Cloning into '.'...
remote: Counting objects: 885, done.
remote: Compressing objects: 100% (410/410), done.
remote: Total 885 (delta 488), reused 842 (delta 464)
Receiving objects: 100% (885/885), 132.08 KiB | 0 bytes/s, done.
Resolving deltas: 100% (488/488), done.
Checking connectivity... done.
2014-07-09 19:58:27,071 DEBUG Checkout ref:hello in /tmp/tmpnum1Rv
2014-07-09 19:58:27,071 DEBUG Executing "git checkout hello" in /tmp/tmpnum1Rv
Branch hello set up to track remote branch hello from origin.
Switched to a new branch 'hello'
2014-07-09 19:58:27,078 DEBUG latest: git://github.com/docker-library/hello-world@e968841b4ed61e28224e750cfbde6d77523f47aa

2014-07-09 19:58:27,078 DEBUG hello-world: latest
2014-07-09 19:58:27,078 DEBUG Cloning repo_url=git://github.com/docker-library/hello-world, ref=e968841b4ed61e28224e750cfbde6d77523f47aa
2014-07-09 19:58:27,078 DEBUG folder = /tmp/tmpmsMIm8
2014-07-09 19:58:27,078 DEBUG Cloning git://github.com/docker-library/hello-world into /tmp/tmpmsMIm8
2014-07-09 19:58:27,079 DEBUG Executing "git clone git://github.com/docker-library/hello-world ." in /tmp/tmpmsMIm8
Cloning into '.'...
remote: Counting objects: 6, done.
remote: Compressing objects: 100% (5/5), done.
remote: Total 6 (delta 0), reused 6 (delta 0)
Receiving objects: 100% (6/6), done.
Checking connectivity... done.
2014-07-09 19:58:27,515 DEBUG Checkout ref:e968841b4ed61e28224e750cfbde6d77523f47aa in /tmp/tmpmsMIm8
2014-07-09 19:58:27,515 DEBUG Executing "git checkout e968841b4ed61e28224e750cfbde6d77523f47aa" in /tmp/tmpmsMIm8
Note: checking out 'e968841b4ed61e28224e750cfbde6d77523f47aa'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by performing another checkout.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -b with the checkout command again. Example:

  git checkout -b new_branch_name

HEAD is now at e968841... Initial commit
2014-07-09 19:58:27,520 INFO Build start: hello-world ('git://github.com/docker-library/hello-world', 'e968841b4ed61e28224e750cfbde6d77523f47aa', '.')
2014-07-09 19:58:28,597 INFO Build success: a6c49877d5d3 (hello-world:latest)
root@60aa8ed6c0f4:/stackbrew/stackbrew# docker run --rm library/hello-world
Hello from Docker.
This message shows that your installation appears to be working correctly.

To generate this message, Docker took the following steps:
 1. The Docker client contacted the Docker daemon.
 2. The Docker daemon pulled the "hello-world" image from the Docker Hub.
    (Assuming that it was not already locally available.)
 3. The Docker daemon created a new container from that image which runs the
    executable that produces the output you are currently reading.
 4. The Docker daemon streamed that output to the Docker client, which
    placed it in your terminal.

To try something more ambitious, you can run an Ubuntu container with:
 $ docker run -it ubuntu bash

For more examples and ideas, visit:
 http://docs.docker.com/userguide/
```