MagicDBUpdater

Added PowerMock support for unit testing.
Added unit test to test PowerMock configuration.
PROTIP: "-noverify" JVM argument must be present when running PowerMock unit tests. This is set in the parent POM, but must be added manually to IDE-run test configurations.
Minor change to MagicGathererDataDownloader and MagicGathererDataObserver to support unit testing.
Added mocking so that testUpdateOfSingleCard() in MagicCardDatabaseUpdaterTest runs as a unit test.
TODO: Complete mocking so all unit tests in MagicCardDatabaseUpdaterTest run without actually having to download data from Gatherer.