Welcome email feature in settings.yml

PT story: https://www.pivotaltracker.com/story/show/73659532
Added feature for welcome email into setting.yml: 
production - enabled
development - disabled
test - enabled

This PR has changes from PR#460, to fix CI for capybara js tests.
good point.  Mailer.send_welcome_message(user).deliver will fail on nil object.  
How about Mailer.send_welcome_message(user).try!(:deliver) ?