Bug 847168 - [User Story] Email Signature

Let's leverage saveAccountDef's third argument being a callback and put the __sendMessage notification in the callback.  That gives us semantics that the caller knows the account update has been persisted to disk (if they care, which they may not.)

I would also add a comment indicating that this will generate an account modification notification too since naive code reading might think saveAccountDef is just doing database gruntwork.