Refactor auth object to { key, secret, passphrase }

[rmm5t]

This more consistent usage allows an AuthenticatedClient instance to be used
in place of an auth object thereby simplifying codebase and general usage patterns.

AuthenticatedClient used to have a b64secret property, but all auth object credentials expected a secret property. By making the two consistent, we gain better consistency, and the codebase can be simplified a bit.

Other considerations
This will likely need to be rebased after #150 is merged.

[rmm5t]

I should also mention that if this refactor is accepted, I'd also like to submit a small change such that the OrderbookSync doesn't necessarily ask for an AuthenticatedClient, but instead just an optional auth object (i.e. { key, secret, passphrase }). Afterall, the authenticatedClient config might conflict with the other OrderbookSync arguments, like apiURI.

(This would still remain backwards compatible, because the property signatures would still behave the same way, but it would free up the library as a whole to move towards better consistency)

[fb55]

Great change. Also in favor of the proposed extension, no reason to need an authenticated client here.