Add pricing from deckbrew.com (refs #147)

* the previous PriceUpdater class has become abstract
* BLPPriceUpdater inherits the old code for blacklotusproject.com
* DBPriceUpdater is a new implementation for deckbrew.com
* add a setting to choose between the two
what will happen here if the card name contains special URL characters like `?` or `&`? Shouldn't you use `QUrl::toPercentEncoding` on the card name?