Add :file option as a new delivery_method for mails

I was searching for all possible options for sending mails and stumbled upon the [edgerails guide](http://edgeguides.rubyonrails.org/action_mailer_basics.html#action-mailer-configuration) which mentioned methods which aren't still part of the documentation.

I updated the `mailer.rb.tt` template with an example.

Matthias