Validate uniqueness of user email address

* Make user.email attribute unique
* Render validations error messages in users/new view

features/step_definitions/user_step.rb

@@ -0,0 +1,12 @@
+Given /^a user with "([^"]*)" email address exists$/ do |email|
+  Factory.create(:user, :email => email)
+end
+
+When /^I register with "([^"]*)" email address$/ do |email|
+  visit new_user_path
+  user = Factory.build(:user)
+  fill_in('Email', :with => email)
+  fill_in('Password', :with => user.password)
+  fill_in('Password confirmation', :with => user.password)
+  click_button('Sign up')
+end