Support other mail services (Google App Engine, Amazon SES, etc)

[techniq]

We discussed this in a Flask-Security issue, but thought I'd put in an issue here to keep track. It would be helpful if you could use the Flask-Mail API to to send mail via a mail service, such as Google App Engine (https://developers.google.com/appengine/docs/python/mail/sendingmail) or Amazon SES (http://aws.amazon.com/ses/). I'm not overly familiar with SES, but it might require using 'boto' to access the API to send email.