Add Mandrill config to .env.example.

Mandrill is a Heroku addon by Mailchimp for sending emails.
Configure actionmailer for dev & prod environments.

Configured to use smtp and Mandrill by Mailchimp.
Update tests to have clearer failures.
Add terminal-notifier-guard.
Add EventSubscriber.
Document Registration.
Set ActionMailer default_url_options for all environments.
Add REPLY_TO_EMAIL to .env.example.
Add RegistrationMailer and templates.