Implement user registration feature

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

features/step_definitions/sessions_step.rb

@@ -21,3 +21,15 @@ When /^I submit valid credentials$/ do
   fill_in('Password', :with => user.password)
   click_button('Sign in')
 end
+
+When /^I follow the sign up link$/ do
+  click_link('Sign up')
+end
+
+When /^I fill in the sign up form$/ do
+  @user = Factory.build(:user)
+  fill_in('Email', :with => @user.email)
+  fill_in('Password', :with => @user.password)
+  fill_in('Password confirmation', :with => @user.password)
+  click_button('Sign up')
+end