Initial effort to add Bitfinex Total Return Swaps capability to XChange.
This implementation currently has tested the raw functions related to 
getting the active swaps order book, sending offers (a swap order in BFX
API terminology), retrieving responses, and cancelling offers. This work
has not been exposed through the generic TradeServices as I would like
to make sure that it is acceptable to add these loan types to the
generic interfaces (and have a bunch of NotImplementedExceptions).