Fixed spec.

Not sure why this used to work, but when I reinstall everything using
ruby-1.9.2-p320 instead of -p180, I got an error about an "Incorrect
date value: '-4712-01-01'." It may have been due to a (less recent)
MySQL upgrade.
Eliminated deprecation warnings from factory_girl.
Fixed specs that weren't cleaning up their messes.

After each spec runs, we remove anything added to the database
during the run. But because organizations were created in a
`before(:all)` hook, our `after(:each)` hook wasn't cleaning them up.

Ordinarily I'd use `before(:each)` but because I didn't write these
tests I'm reluctant to change their logic much. So we use
`build_stubbed` instead of `create` to simulate a saved organization
without actually putting it in the database. And I did move the
payment that was created to a `before(:each)`.
Installed Poltergeist for JavaScript testing.
Added test for signup with discount code.