Allows any class extending the Mailer trait to accept the same instances of From(), To(), Subject(), etc.
… added doco.
This allows users to compliment or override the default logic used to determine the run-mode (ie. dev/test/staging/etc) in absence of a "run.mode" system variable. An example use case would be a user using test framework that Lift doesn't know about, and it undesirably running in development mode.