LogStasher::MailerLogSubscriber successfully attaches itself to ActionMailer and logs e-mail metadata.
Added plumbing between LogStasher and the rails app.
Moved SampleMailer to a separate file.
Fixed failing test for ActionMailer/Rails with no templates.
Request context functionality - it's a thread-safe dictionary available globally, and initialised during `process_action`. It is used primarily to provide context to ActionMailer log subscribers.

The default field added is request_id which is automatically generated by rails.

Additional fields can be added by using:

LogStasher.add_custom_fields_to_request_context do |fields|
  fields[:key] = value
end

The block above is executed in a callback, after a request has been completed.
Merge pull request #40 from jasiek/action-mailer-custom-fields

Action Mailer logging + Request context