Implement user registration feature

* Add users/new action and view
* Add users/create action
* Add link to sign up in sign in page

spec/controllers/users_controller_spec.rb

@@ -0,0 +1,44 @@
+require 'spec_helper'
+
+describe UsersController do
+  describe 'GET new' do
+    it 'assigns a new user as @user' do
+      get :new
+      assigns[:user].should be_a_new(User)
+    end
+  end
+
+  describe 'POST create' do
+    context 'whith valid params' do
+      it 'creates a new user' do
+        expect {
+          post :create, :user => Factory.attributes_for(:user)
+        }.to change(User, :count).by(1)
+      end
+
+      it 'signs the user in' do
+        post :create, :user => Factory.attributes_for(:user)
+        controller.current_user.should_not be_nil
+      end
+
+      it 'redirects to the home page' do
+        post :create, :user => Factory.attributes_for(:user)
+        response.should redirect_to(:root)
+      end
+    end
+
+    context 'whith invalid params' do
+      before { User.any_instance.stub(:save).and_return(false) }
+
+      it 'assigns the user as @user' do
+        post :create, :user => {}
+        assigns[:user].should be_a_new(User)
+      end
+
+      it 'renders the new template' do
+        post :create, :user => {}
+        response.should render_template('new')
+      end
+    end
+  end
+end