Implement user registration feature

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

app/views/sessions/new.html.haml

@@ -4,3 +4,4 @@
   = f.label :password
   = f.password_field :password
   = f.submit 'Sign in'
+= link_to 'Sign up', new_user_path

app/views/users/new.html.haml

@@ -0,0 +1,8 @@
+= form_for(@user, :url => users_path) do |f|
+  = f.label :email
+  = f.text_field :email
+  = f.label :password
+  = f.password_field :password
+  = f.label :password_confirmation
+  = f.password_field :password_confirmation
+  = f.submit 'Sign up'