Scryfall requests should be rate-limited

[Andrew Moore]

From the API documentation:

Rate Limits and Good Citizenship
We kindly ask that you insert 50 – 100 milliseconds of delay between the requests you send to the server at api.scryfall.com. (i.e., 10 requests per second on average).

Submitting excessive requests to the server may result in a HTTP 429 Too Many Requests status code. Continuing to overload the API after this point may result in a temporary or permanent ban of your IP address.

This will need to be applied to search_cards() and get_cards_by_name().

[Andrew Moore]

Think this can be closed for now, might re-open if I want to pass the responsibility of handling rate-limiting by the user.