This pull request includes two patches:
NB: The change in using key from 'method' to 'delivery_method' should not break settings for existing installation because they are already broken and use default :sendmail settings due fallback on the 'server_settings' assignment exception.
Reads from mail.yaml value of delivery_method and settings for each
method supported, and assings them to ActionMailer options.
issue #751, mail configuration example rewritten