Mailer inner classes

Allows any class extending `trait Mailer` to accept the same instances of `From()`, `To()`, `Subject()`, etc.
This in turn allows for easier testing & mocking of `Mailer`.

(Supersedes #1468 which was against the obsolete `lift_26_dev` branch instead of `master`.)

https://groups.google.com/forum/#!topic/liftweb/HtBCb1XmmU4