Support for multiple API keys

[Valian]

First of all, thank you a lot for creating this wrapper! It helps me a lot.

I'm making an application that acts on behalf of multiple users. Currently api key and secret are global, stored in config. I'd love to be able to make requests using different credentials.

If I'll find time I'll try to create a PR, but just wanted to create an issue to let you know such feature would be very useful :)

[dvcrn]

Hey! There is a PR open on adding support for passing apikey/secret as opts instead of global vars, can you check it out? #107

Reason it's not merged yet was that the initial reporter noticed some problems with it, see this comment: #105 (comment)

Could you give this branch a try and report if it works for you?