Mtgjson importer (fixes #84)

This PR makes the needed changes to initially support downloading card sets from mtgjson.com.
The only real problem i found till now is that split cards are imported as 2 different cards.
While this change is surely not ready yet for merging, I'll leave this open for testing and further discussion, so that it can be improved... well it's better to have a slightly broken importer than no importer at all :)