Mailer Functionality

I was playing around with your Selfstarter App as kind of a first project. Please do excuse the code if it is a little messy. 

In terms of changes I have made / added:

- Changed the Gemfile to include Rails 4.0, Ruby 2.0, and delayed_job
- Added mailer functionality. Config currently set to MandrillApp but you can adjust based off which mailing client you prefer
- Implemented mailer functionality by clicking 'Remind me' button on checkout page