Bittrade-kraken-rest

Added

- Edit order endpoint

Changed

- The sign function provided as example, handles json payloads such as cancel batch

Changed

- Example signature method updated to reflect fix for json body
- `prepare_private` accepts a `json` dict; if provided, content type header is set to application/json
- batch cancel orders uses that new json argument to work
- batch cancel orders reverts to regular cancel if only one id is passed (Kraken fails otherwise)

Changed

- Cancel order batch is now exported on root module
- Results of requests mapped throught `send_and_map_result` aren't printed anymore; they are logged at the debug level

Added

- Cancel order batch

Changed

- Nonce uses nanoseconds

Changed

- `oflags` not sent when not sent (fixes order creation error)