Add authentication and User model

* Add User model
* Add SessionsController
* Add password authentication on User
* Request authentication for all actions except sign in
* Add some helpers for ApplicationController
* Update features to work with mandatory authentication

spec/controllers/sessions_controller_spec.rb

@@ -0,0 +1,40 @@
+require 'spec_helper'
+
+describe SessionsController do
+  describe 'GET new' do
+    it 'responds successfully' do
+      response.should be_success
+    end
+  end
+
+  describe 'POST create' do
+    context 'when the user submit invalid credentials' do
+      it 'renders the new template' do
+        User.stub(:authenticate).and_return(false)
+        post :create,
+          :session => Factory.attributes_for(:user, :password => 'WRONG')
+        response.should render_template('new')
+      end
+    end
+
+    context 'when the user submit valid credentials' do
+      let(:user) { Factory.create(:user) }
+      before do
+        User.stub(:authenticate).and_return(user)
+      end
+
+      it 'signs the user in' do
+        post :create, :session => Factory.attributes_for(:user)
+        controller.current_user.should == user
+      end
+
+      it 'redirects to the home page' do
+        post :create, :session => Factory.attributes_for(:user)
+        response.should redirect_to(:root)
+      end
+    end
+  end
+
+  describe 'DELETE destroy' do
+  end
+end