[enhancement] Make async requests

[tuloski]

Hi, would it be possible to make all the request async, possibly with promises?

[KevinJue]

@tuloski you got the solution here https://github.com/krystianity/node-bitstamp

[askmike]

@tuloski it follows default node callback style. You can use a tool like pify to easily turn this into a promise.

@KevinJue that's a completely different project.