Add TransferWise as a cashout method

TransferWise is currently the best (lowest fee) option for USA to EU transfers. They also have an open API, and no extra fees for using the API. https://api-docs.transferwise.com/#transferwise-api

Paypal takes over 10% in total fees (Paypal transfer fee + fee hidden in exchange rate). This on top of Bountysource's 10% adds up.
BTC is way too volatile, and has regulatory issues (you may have to pay VAT on it, since it is classified as property in some places).
Checks and international wire orders are expensive.

Meanwhile, TransferWise gives you the proper exchange rate, and charges low fees (1%, 1-3$ min, depending on currency). The recipient does not need a TransferWise account, they just put in their bank info.

Start with the linked TransferWise API documentation to understand the proposed cashout flow, fees, and recipient requirements. The issue does not name repository files, tests, or an entry point; completion would mean supporting TransferWise as a cashout method alongside the existing options.

Written by the indexing model from the issue text.