Add a shortcut method for setting files.

Hi there,

I think it's great if rails_config provide setting file paths because I want to use it for rake task and unicorn.
For example, in my ```config/unicorn.rb```

```ruby
RailsConfig.load_and_set_settings(RailsConfig.setting_files(File.expand_path("../", __FILE__), ENV['RAILS_ENV'] || 'development'))
```

What do you think?