Add support for a .spring.rb in the project root that always gets loaded early

Our own unique little snowflake of a Rails App has multiple facets (www.sauspiel.de vs www.skatstube.de vs www.fuchstreff.de) that are loaded depending on an environment variable `APP`. So when developing we run multiple instances of `rails server` with different `ENV['APP']`.

We recently updated to Rails 4.1.0.rc2 and wanted to use spring in our development workflow.

In order to make this work with spring I had to monkey patch `Spring::Env`, but there was no good point to hook into spring. That's why I added support for a `.spring.rb` in the project root that is loaded before calling the spring `Command`, so it's a good place to modify spring for our own unique needs.

In our concrete case it looks like this:
```ruby
ENV['APP'] ||= begin
  config_file = File.expand_path('../.app', __FILE__)
  File.read(config_file).chomp if File.exists?(config_file)
end
abort "No or invalid APP, run something like this instead: APP=sauspiel #{$0} #{ARGV.tap(&:shift).join(" ")}" unless %w(bummerl fuchstreff sauspiel skatstube).include?(ENV['APP'])

abort "You're using an unsupported spring version. Check that it works and then update the check in .spring.rb" unless Spring::VERSION == "1.1.2"

module Spring
  class Env
    def app_name
      ENV['APP']
    end

    def application_id
      Digest::MD5.hexdigest(ENV['APP'])
    end
  end
end
```

An alternative, instead of introducing yet another config file that gets loaded at another point during startup, would be to always load `config/spring.rb` early and use `Spring.after_fork` or another (to be added) callback for stuff that has performance implications and should only run once.

But I saw in the changelog that the point where `config/spring.rb` gets loaded was already changed once "so you can require stuff from other gems there without performance implications". (16f5a24b776ac70e79835f9c5621d150ad5d0dc3 / #79)

I'd be happy to make such a change if it means we can use spring in our application.