Support email attachments

@glasser I'm not sure what needs to change in the _.each for attachments in packages/email/email.js to support binary attachments, so I updated comment.

With some guidance, I'd be happy to do what is needed to support binaries.

Once code supports binary attachments, I'll update email_tests.js. Considering we'd then be supporting both text + binary attachments, should we make make new test examples, or would you prefer just adding to the test one ascii and one binary attachment?