development mode

Add "Configuring Middleware" section to config guide, starting with mentioning what every bit of middleware does.

There is no RAILS_GEM_VERSION environment variable or constant for Rails 3, since this is now managed by Bundler

* 'master' of github.com:lifo/docrails:
  Fix missing word in ActionMailer::Base documentation.

Update ActionMailer documentation to not use deprecated template_root method as documentation, but rather raise_delivery_errors method