Switch to enum for contract, new futures impls
Inital commit for futures api
Renamed
Move FuturesPrompt to OkCoin and rename to FuturesContract
Separate Market,Trade,Account impls for Futures
Revert
Fixed okcoin test
Renamed OkCoinTradesDemo
Merge pull request #750 from jheusser/improveokcoinfutures

OkCoin: new API, Futures support