Coinffeine/coinffeine — pull request #201 by sortega (2014-06-26 10:37:17+00:00)
Title: Refactor client.exchange.Exchange to make it similar to MicroPaymentChannel

**Description:** refactor client.exchange.Exchange to make it similar to MicroPaymentChannel. To do so, remove the UserRole class hierarchy and extract the payment processor related responsibilities.
**Testing:** tests pass.
**Coverage:** about the same.