ignoring Gemfile.lock caused problems with Heroku
[6.1.1] generate users model
[6.1.2] add annotate gem and annotate user model
[6.2.1] create user model test
[6.2.2] create presence of validations for user name and email
[6.2.3-5] create validation rules for user name and email

* name length
* email format
* email uniqueness - including database unique index
[6.3.1] add password_digest to user for authentication
[6.3.2-4] create password validations