Bug 847168 - [User Story] Email Signature

This wants a jsdoc comment.  You don't have to break out all of the fields specifically, but convey that the mods is a dictionary of values to change and that anything that's omitted will be left as is.  And that the result of this call is that a notification will be sent out updating all MailAccount instances for this account and thereby this MailSenderIdentity too.