Preview

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WARNING!!! ACHTUNG!!! ATTENZIONE!!!
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We need to put some integration tests that directly hit Heroku. This needs quite a bit of thought but, nevertheless, we need to address it

@exalted: Your feedback on the current design is very welcome. Especially, what do you think about this piece of code?

```ruby
origin      = Heroku::Application.from_name("catchup-web")
destination = Heroku::Application.from_name("preview-app-1234")

destination.destroy
origin.fork_into(destination)
destination.deploy_branch(tarball)
destination.run_command("rake db:migrate")
```

Even though I am really proud of the readability this has, I would be happy to hear any (positive) critic from you.
I personally have no real issues with this. As an alternative `Heroku:Application` could do this internally, so maybe instead creating two `Heroku:Application`s, a single `Heroku:Application` could have `origin_app` and `fork_app` properties, or something like that?