* Added Card#used_count jand Card#available_count
    - added rspec tests too
* JSON for Card index and show now includes available_count
* Delete confirmation for deleting a card shows the number of decks the card is used in
* Deleting a card or deleting a deck now destroys the card/deck association (used to leave them dangling)
* Card show now shows the number of available cards and which decks the card is used in
* Deck builder now makes use of Card#available_count to prevent adding cards to a deck for
  which there are no (0) available copies.
* Implemeted a couple of pending Deck tests
    - can count number of cards in main deck
    - can count number of cards in side deck