Add user authentication

- Added user model
- Added user/new viewj
Format css for User/create form
Add messages to application.erb.html
Add authentication

-use has_secure_password method
Finish authetication

- create authenticate method in User that uses 'find_by_email'
- fix features/user_logs_in_spec to use new terminology
- Add login link to Welcome page
- Add sessions resource with 'log_in' for path allowing for use of
  'log_in_path'