Send the X-MC-Tags header to Mandrill

[seanh]

Whenever sending an email always include an X-MC-Tags header identifying the type of email (activation, password reset, reply notification, etc).

This will enable easy filtering by type of email in Mandrill's dashboard, allowing us to easily view stats like number of emails sent, bounce and failure rates, etc by type of email.

This can be implemented by adding a mandatory tag argument to EmailService.send()