Added whenever gem & scheduled task for email_job
/signup POST now schedules send of intro e-mail

Added a new config/site.yml option named 'domain.' The intro e-mail
includes the site's domain. This might be removed in favor of getting
the domain from the /signup POST request.

Test code now deletes all Users, Bets, and EmailJobs after each example
group. The test for /signup POST creates an EmailJob, and this caused
the test in spec/rakefile_spec to fail because it initially expects 0
EmailJobs.