Rename client.exchange.Exchange to ProtoMicroPaymentChannel
Make exchange and role available on MicroPaymentChannelActor
Remove UserRole hierarchy
Move pay responsibility out of ProtoMicroPaymentChannel
Move payment validation from ProtoMicroPaymentChannel
Remove some payment-related leftovers
Merge pull request #201 from sortega/issues/41bis

Refactor client.exchange.Exchange to make it similar to MicroPaymentChannel