Implement Forgot Password Feature

This pull request adds support for user password resets via an insecure token generation and reset mechanism. Additionally, RailsGoat leverages the use of MailCatcher, a simple SMTP server that intercepts the password reset email with the user's corresponding reset link, and displays it in a web interface.

- [x] Create ActiveMailer `user_mailer`
- [x] Configure RailsGoat to use MailCatcher
- [x] Implement password reset controller
- [x] Update views and README file
- [ ] Complete mechanism to update password after successful reset

The final component to close Issue #68 is to complete the password update method that will store the newly created password after a successful reset.