Adding email service: Mandrill

Mailgun and Sendgrid are good solutions for email services, but so is Mandrill (hence the PR).

This will add an extra option to the generator and do the correct replacements within the user.js and contact.js controllers when selected. I also fixed an issue within the generator where the second nodemailer instance, within the user.js controller, was not being updated by the generator.

Going forward: The mandrill service will need a test account similar to how you have accounts setup for mailgun and sendgrid.

Let me know if you need anything changed/updated.

Cheers!