Gocardless-pro

* Fix bug in setup.py preventing installation for some users (see 9)

* Allow passing in custom headers (e.g. Accept-Language)
* Add `payments_require_approval` to Mandates
* Add `deducted_fees` to Payouts

* Fix duplicate bank account error string representation (4)

* Exclude tests from the installed package

* Add new API attributes to resource classes:
* `BankDetailsLookup.bic`
* `Creditor.default_sek_payout_account`
* `Customer.language`
* `Customer.swedish_identity_number`
* `Event.organisation`
* `RedirectFlow.customer`
* `Refund.reference`

* Fix support for endpoints that use "data" as the request envelope
* Make exception string representations more detailed