add a function into Shotgun_api3 for emailing

We'd like to be able to email certain people, from no-reply@shotgunstudio.com, based on different rules. Sometimes, this would be the best way to deliver information meant only for a user (instead of creating a note and then restricting permissions). Sending people reminders to log hours (and including their logging statistics) is one example of transmitted data that is better as an email, instead of a note.