Project generator

Mix task that generates a new Phoenix application.

In `template` directory we have clean application with default router and one example controller. In future that controller action would render a template with some useful getting started information. I believe it makes more sense to have something small and real that works out of the box than having `public/index.html` that you have to delete. I would also like to include examples of routing DSL in default router, but since we are in transition with routing DSL I decided to postpone that.

Usage:
```console
mix phoenix.new you_app /path/to
```

Would love to hear your comments.