Add psych as development dependency

* Eliminates warnings and errors when psych is installed
Add Clearance::BackDoor for signing in quickly in tests

http://robots.thoughtbot.com/post/37907699673/faster-tests
Make PasswordsController#create case-insensitive

* Centralize email normalization logic in `User.normalize_email`.
* Implement `User.find_by_normalized_email`.
Removes duplicated sign up and forgot password links
Move password email delivery to private method

* The password email delivery can now be overridden