Add site generator

Just like `rails new appname`, this branch lets you do:

``` bash
hyperloop new sitename
```

And then you get:

```
sitename/
├── Gemfile
├── app
│   ├── assets
│   │   ├── javascripts
│   │   │   ├── app.js
│   │   │   ├── current-time.coffee
│   │   │   └── jquery.js
│   │   └── stylesheets
│   │       ├── app.css
│   │       ├── bootstrap.css
│   │       └── main.scss
│   └── views
│       ├── _partial.html.erb
│       ├── about.html.erb
│       ├── index.html.erb
│       └── layouts
│           └── application.html.erb
└── config.ru
```

I followed [this guide](https://github.com/radar/guides/blob/master/gem-development.md#crafting-a-cli) (minus the Cucumber and Aruba parts) to get this going. Probably a lot of improvements I could make, but for now I figured it was important to just get this going and not over-engineer it.

/cc @jessicard @fabianperez