Provide a configuration option to set the rails root

This will solve #36 by providing an option to configure the rails root:

**./config/spring.rb**
```ruby
Spring.configure do
  config.application_root = './spec/dummy'
end
```

This is not a final version and there are some hacky things I would like to discuss:

1. I'm not sure if the `Env` is the right container for the configuration. It makes it necessary to pass it through all the objects.
1. I'm not sure if we should rely on `ActiveSupport::Configurable` or if we should just roll our own configuration object.
1. I did not write any tests. I'd like to bundle an engine with a custom configuration to test it against a real world example. Do you think this would be too much overhead?