Open Bank Withdrawal

[Fulk33]

Could we get this API call (and maybe "withdrawal status" and "cancel withdrawal") added to the client? Would be nice to have.

[Fulk33]

I created a new branch with the changes (https://github.com/Fulk33/bitstamp-python-client/tree/bank_withdrawals). I don't know how I could write tests for them though. Any ideas?