Add Makefile for publishing static web page to GitHub Pages.

Description
-----------

[GitHub Pages](https://pages.github.com/) provides a static web page hosting for free. With this [Makefile](http://www.gnu.org/software/make/manual/make.html), you can easily publish your game.


Usage
-----

Just move to your repository location, then enter `make` command.

```bash
$ cd /path/to/soongbu-highway
$ make
rm -rf /tmp/soongbu-highway
git clone https://github.com/GDG-SSU/soongbu-highway.git /tmp/soongbu-highway
Cloning into '/tmp/soongbu-highway'...
remote: Counting objects: 288, done.
remote: Compressing objects: 100% (141/141), done.
remote: Total 288 (delta 84), reused 286 (delta 83)
Receiving objects: 100% (288/288), 978.38 KiB | 386.00 KiB/s, done.
Resolving deltas: 100% (84/84), done.
Checking connectivity... done.
cd /tmp/soongbu-highway; \
        git checkout gh-pages || git checkout -b gh-pages; \
        git rebase master; \
        git push -f origin gh-pages
error: pathspec 'gh-pages' did not match any file(s) known to git.
Switched to a new branch 'gh-pages'
Current branch gh-pages is up to date.
Total 0 (delta 0), reused 0 (delta 0)
To https://github.com/GDG-SSU/soongbu-highway.git
 * [new branch]      gh-pages -> gh-pages
rm -rf /tmp/soongbu-highway
```

Don't worry, this Makefile will:

1. Clone your repository into `/tmp/soongbu-highway`.
2. Checkout branch named `gh-pages`. If not exists, create it.
3. Rebase `master` branch.
4. Push branch `gh-pages` to remote.

After done, you'll able to play your game here: [http://GDG-SSU.github.io/soongbu-highway](http://GDG-SSU.github.io/soongbu-highway).


Example
-------

You can see an example on my GitHub page: [http://devxoul.github.io/soongbu-highway/](http://devxoul.github.io/soongbu-highway/)