Update node-bitx to latest BitX API spec: add support for multiple currency pairs and the new balance method.

Hi

The BitX API has recently been updated:
https://bitx.co.za/api#changelog

Changes include support for multiple currency pairs and a new "balance" method. The old "getlimits" method has been deprecated.

This PR updates node-bitx to the latest spec.

By the way, what is the license for node-bitx? If you haven't already chosen, I'd recommend MIT.

Regards,
Timothy