Use ArtifactID instead of int when possible
Implement CArtHandler::isBigArtifact and CArtifact::isTradable
Use isTradable and fix issue 1392

This change make it possible to sell spell scrolls in Artifacts Merchant.
Use switch for CArtHandler::isTradableArtifact
Merge pull request #69 from ArseniyShestakov/artifactsImprovements

Thanks, looks great now.