SteamCardExchange links for badges

SteamCardExchange is quite convenient as it has the trading card, background
and emoticon images for all games available.

The few badges not associated with any game aren't on SteamCardExchange, thus
they're not explicitly linked. The URL format differs for those too - /badges/1, not
/gamecard/1, so they're not given any additional links.
Merge pull request #90 from mabako/steamcardexchange

Adds SteamCardExchange links for badges