anchor.diff:  
   Pass `guest` value as username/password.
   
  +#### Web authorization
  +
  +Grant type: `authorization_code`
  +
  +First get authorization using `authorize` endpoint and pass it via `code` attribute. Also pass
  +`redirect_uri` which should be the same as passed to the `authorize` endpoint.
  +
   #### Login with email and password
   
   Grant type: `password`
  
positive.code: and pass it to the token endpoint via `assertion` attribute.
  
  + Attributes
      + client_id: android (string, required) - Client app identifier. One of: *android*, *ios*, *web*.
      + grant_type: password (string, required) - OAuth2 grant type. One of: *password*, *assertion*.
      + scope: user (string, optional) - Access scope.
      + username: gediminas@vilnius.lt (string, optional) - User's email address. **Required for password grant type**.
      + password: secret (string, optional) - User's password. **Required for password grant type**.
      + provider: google (string, optional) - Assertion provider. One of: *google*, *facebook*, *viisp*. **Required for assertion grant type**.
      + assertion: token123 (string, optional) - Google/Facebook/VIISP token. **Required for assertion grant type**.
  
  + Request Create a token using email/password.
  
              {
                  "client_id": "android",
                  "grant_type": "password",
                  "scope": "user",
                  "username": "gediminas@vilnius.lt",
                  "password": "secret"
              }
  
negative.code: ```ruby
  require 'oauth2'
  
  client = OAuth2::Client.new('android', nil, site: 'https://tvarkaumiesta.lt/api/oauth')
  token = client.password.get_token('guest', 'guest')
  
  puts token.token
  