Adding JWT gem and creating models / migrations

- added the JWT gem for JSON Web Token manipulation
- created the models: user, link and tag
- migrations created for the models described above and also a join table migration for links / tags
Implementing authentication and registration

- added AuthToken module to issue / validate the JWT token
- using has_secure_password for password storage
- created controllers: auth and base auth
- added routes for registration and authentication
Merge pull request #13 from ramirovjr/master

Implementing authentication, registration and JWT