added bitcoin support

[ryan-lang]

trafficstars

added create_bitcoin_receiver(), get_bitcoin_receiver(), and list_bitcoin_receivers(). Also updated the documentation on post_charge(), which can now accept a Bitcoin receiver id (analogous with a card token.) addresses #63

[ryan-lang]

I should also note that the new Stripe api encourages using the parameter "source" rather than "card" for cards, tokens, and bitcoin, but I decided to leave that alone for now since that's a more substantial update.

[andrewsolomon]

Unfortunately this breaks unit tests for me because I don't have a USD bank account

ok 79 - charge was paid
Error: invalid_request_error - For 'bitcoin_receiver' payments, we currently require your Stripe account to have a bank account in the currency of the payment (usd) On parameter: currency
# Tests were run but no plan was declared and done_testing() was not seen.
# Looks like your test exited with 255 just after 79.
Error: invalid_request_error - For 'bitcoin_receiver' payments, we currently require your Stripe account to have a bank account in the currency of the payment (usd) On parameter: currency
Dubious, test returned 255 (wstat 65280, 0xff00)
All 79 subtests passed