Replace `fee` with `balance_transaction`

This changed in Stripe API 2013-08-13

We don't have functions for reading balance_transaction(s), but
Stripe returns a txn_id instead of a nested transaction, so you
can log the ID and read it later.
Remove `active_card` fields

active_card is no longer returned as of API 2013-07-05

Cards are now returned as a list with key 'data' -- we can figure
out how to read those later.
Note where Stripe API changes exist online

So I can stop asking Google where to find this page, let's just
put it right here in the README.

Not all Stripe API changes require code changes because this Stripe
client doesn't support all Stripe features or values of returned
data types.

All eunit Stripe tests pass as of right now, 2131 EST 2013-12-18.