add basic invitable framework

gem 'devise_invitable', '~> 1.3.4'        # invite users
bundle install
rails generate devise_invitable:install
rails generate devise_invitable
rake db:migrate
make invite users appear to work - closing (#27)

rails generate devise_invitable:views
app/views/layouts/application.html.haml
+              %li= link_to 'Invite a friend', new_user_invitation_path
config/environments/development.rb
+  config.action_mailer.default_url_options = { :host => 'localhost:3000' }
+  config.action_mailer.delivery_method = :test
Merge pull request #133 from nbirnel/invite-user

Invite user