Non-singleton mailer settings

[ericgj]

See what you think of this. It's a bit ugly under the hood, but this Newman::Mailer is a drop-in replacement that circumvents Mail's horribly constraining config system.

It's a first step towards being able to 'multiplex' mailboxes, i.e. run an app on several mailboxes at once.

[practicingruby]

ooh this looks nice. I'm going to pull this in and make some small revisions.