Update README.md
Enabling sign in and email confirmation with devise.

* Added email_spec gem helper methods
Generating unique API Token on user creation.

In order to allow API requests each user needs a token. Now indexing
on `api_token` and making sure each user has a unique one.
Adding rating table and seed data.

Necessary for future tests of API and website. The data needs to
reflect the real-world data as much as possible. Possible side
effects include slower testing because we reload the data after
every DatabaseCleaner deletion.