Setup user login/logout (#21)

Reviewed-on: #21

app/models/session.rb

@@ -0,0 +1,22 @@
+class Session
+  include ActiveModel::Model
+
+  include ActiveModel::Attributes
+  include ActiveModel::Validations
+
+  attr_accessor :user_id
+
+  attribute :email, :string
+  attribute :password, :string
+
+  validates :email, presence: true
+  validates :password, presence: true
+
+  def save
+    user = User.authenticate_by(email: email, password: password)
+
+    @user_id = user && user.id
+
+    user.present? && self || nil
+  end
+end