Create UserPreferences Model. Accessible by :preferences

Associate User model with UserPreferences
Association is built upon creation of a user

:minutes_before_remind_at  to have a default value of 15.
Create UserPreferences Model. Accessible by :preferences

Associate User model with UserPreferences
Association is built upon creation of a user

:minutes_before_remind_at  to have a default value of 15.
Merge pull request #5 from tylerkern/add-user-preferences

Add user preferences