Moved json response method to test helper.

It DRYed test code a little.
Added simple authentication via token system.

We can't use devise since we are building API. This simple approach is good enough for now.
Added BCrypt to Gemfile.

We're using #has_secure_password inside User model which depends on bcrypt gem. That's why it must be added.
Added authentication controller with tests to provide means for authentication.

We are sending authentication in X-Authentication-Token. It is convenient even for mobile devices.