Bunch of changes at once, because I'm a Java n00b:

* Put our main() on an Application class
* When instantiating Application, optionally give it an AcmeUI
* Test that the app can create a completely trivial ad (because
  there's a completely trivial AcmeAd class)
* Test that after making an ad, we still have it (because the app
  has an AcmeAds list)

If we make an AcmeUI, we get a JFrame on screen. Cool, I'll add
more Swing bits later. Tests don't have to make an AcmeUI, so I can
proceed to write fast tests for more application behaviors.